Dual Mode Unit

A Real Time Strategy unit type. It has two different weapons, functions, or abilities that can be switched between manually, or chosen automatically depending on the target. Sometimes you may have to research the secondary form before the unit can access it.

There's always some kind of trade-off in these cases. Often, a unit must park itself to use its secondary mode, but gains extended range and attack power as a result, or it loses the ability to target ground units for the power to hit air. Sometimes, a unit can even switch from a land form into an air or sea form. Other times, the secondary mode might just be an outright Super Mode, with the catch being some new contrived weakness, a time limit, or even a high resource or XP cost to obtain it.

It's a step up from Crippling Overspecialization, but beware. It may just end up with two different crippling overspecializations. This type of unit might be a Military Mashup Machine, but it doesn't have to be.

Video Game Examples :

 * Starcraft really got the ball rolling. Nearly every Terran unit has some kind of special ability, but the one that fits the pattern best is the Siege Tank. It goes from a mobile tank with a decent cannon and no minimum range to a fixed artillery gun with a powerful, long range attack. Other units have different attack modes for air or ground targets, but that mode switch is automatic.
 * Starcraft II features the Viking, a Transforming Mecha that can switch between a ground-based and aerial unit.
 * Another good example is the Lurker. Traditionally the Zerg burrow units to hide them from the enemy, but the Lurker - which is completely unarmed in its standard walking mode - turns into a horrible slice-and-mince troop blender when burrowed.
 * The Command & Conquer originally used this sparingly. The first example came in Tiberian Sun, where one of Nod's tanks could deploy into a stationary turret (and undeploy into a tank again, if necessary). This was expanded upon in Red Alert 2, where Allied GI units (their basic infantry) could deploy behind sandbags to increase their range, power, and defense at the cost of movement, and the IFV, which could carry a single infantryman, and swapped out it's missiles for a different weapon depending on which trooper was in it. The Expansion Pack added Guardian GIs, whose deployment allowed them to attack armor and Anti-Air. Red Alert 3 took it to the next level and specifically designed every unit to have two modes. The special abilities range from weapon switches like Conscripts that can switch from rifles to molotov cocktails; to units that deploy into expansion structures. The Empire of the Rising Sun is the real standout. They are the only faction that does not have an airbase structure because all of their air units are transformed from sea units, Humongous Mecha, or infantry with jetpacks (who either shoot rockets or unit-disabling beams).
 * Spin off studio Petroglyph's game Universe At War had the Masari, whose entire military doctrine relies upon this trope. All units and buildings could switch between Light and Dark Mode. Light Mode increased the damage and sight range of units, while Dark Mode strengthened their armor instead (air units would also be grounded by switching). Even the superweapon of the Masari behaved differently depending on its mode.
 * Star Control had two: First, the Mmrnmhrm X-Form, a swing-wing ship with two modes, one was slow but nimble and armed with short range lasers while the other was fast, armed with (very) long range slightly guided rockets and all the nimble turning speed of a anvil. Second, the Androsynth Guardian which could turn from a slow ship into a destructive comet, maneuverability and weapons for speed and the ability to ram.
 * All units from Ground Control 2 have two functions. The basic infantry freeze in place, but switch from their rifles to a more powerful rocket launcher. The APC switches from an anti-infantry machine gun to a concealment-granting smoke grenade, etc.
 * Most Transforming Mecha in the Super Robot Wars series are given dual modes when playable. For example, the Valkryies of Macross, or the Original Generation mecha Cybuster, Real Personal Trooper Type-1 (R-1), and Valhawk, which are often faster in their flying modes, but usually have more attack options as mecha. The Grungust has this, as well as another alt-mode (Gust Lander) that transforms it into a tank.
 * Almost all units in Star Wars: Empire At War and its expansion, including heroes. Most of the time, this helps to offset Crippling Overspecialization. Artillery units have to deploy to fire their main weapons, AT-AT's can deploy stormtroopers, Airspeeders can use their harpoons and tow cables to trip up AT-AT's (except Blizzard 1, and sometimes the move doesn't work), Immobilizer 418 cruisers (aka Interdictor Star Destroyer's or Interdictor Cruisers) must manually activate their Gravity Well Generators, and also have a missile/torpedo-jamming ability, Crusader-class corvettes can shoot down incoming missiles/torpedoes, etc. etc.
 * Quite a few in the Nintendo Wars series. For example, Tanks and Mechanized Infantry units usually carry an anti-vehicle weapon (Cannons for the Tanks, Bazookas for the Mechs) and an anti-infantry weapon (Machine guns and rifles.) The anti-infantry weapons can be used on vehicles if the anti-vehicle weapon is out of ammo, but it does considerably less damage, and in earlier games in the series, it also has limited ammo.
 * Unreal Tournament 2004 brings us the Leviathan, a land-based gunship with a rapid-fire homing missile launcher you really don't need to aim and four individually-controlled turrets. Then it unfolds, revealing a mantis-head turret that fires a beam, producing an explosion larger than even the infamous Redeemer missile. With infinite ammo, no less. The catch is, of course, when unfolded, you're EXTREMELY vulnerable to aerial vehicles like the Raptor and Cicada.
 * The Cybran Nation in Supreme Commander has a T2 naval unit that can sprout legs and walk on land. This transition takes a significant amount of time.
 * Warcraft 3 has several examples:
 * The Night Elf Druids can, after you have researched the proper upgrades, transform into animal forms that function as very different units. The Druid of the Claw is a healer and support caster in default Night Elf form, but becomes a powerful heavy melee unit in Bear Form. The Druid of the Talon is a disruptive caster in Night Elf form, but becomes a flying scout and anti-air unit in Crow Form.
 * And in the expansion, they can be upgraded to be able to cast their allied-damage-enhancing / autocast-enemy-armor-removing spell while transformed.
 * Night Elf Archers are basic ranged units that need to be produced en masse to be useful, Hippogryphs are flying creatures that can only attack other air units. There's an upgrade that allows an Archer to ride a Hippogryph, losing the stronger anti-air attack in favor of attacking both ground and air, not to mention having higher health. In the expansion, it can also be reversed after a cooldown, in case a large force of air units is encountered.
 * The Undead Gargoyle is a light flying unit regularly, but can be upgraded with Stone Form. In Stone Form, it cannot move or attack and becomes a ground unit, but becomes extremely tough and regenerates quickly. The Undead Crypt Fiends also have a similar ability, Burrow, which makes them incapable of moving or attacking, but makes them invisible and allows them to regenerate quickly.
 * The Tauren Spirit Walker can switch between Ethereal and Corporeal Forms. Although it remains essentially the same unit(a support caster) in both forms, it cannot attack and becomes immune to physical damage in Ethereal Form, although it becomes more vulnerable to magic.
 * The Human Footman unit can go into Defend mode, which basically makes it much tougher against Piercing damage (most ranged attacks), but makes it move much more slowly. The Footman remains the same basic light melee unit in both modes, so it's not a very drastic mode change.
 * The Goblin Tinker, a neutral Hero unit, can gain the ability to enter Robo Goblin mode upon reaching Level 6. Robo Goblin mode is tougher, stronger, gets devastating bonus damage against buildings, and becomes immune to most offensive spells, but it doesn't regenerate like a normal Hero would and cannot be healed by conventional methods, but instead needs to be repaired like a building.
 * Some of the Night Elves' buildings are actually sentient trees called Ancients, which can be 'Uprooted', transforming them from buildings into slow-moving melee units. While Uprooted, the Ancients lose their regular building functions(training or upgrading units, harvesting gold, etc.) and lose their Fortified armor type, but gain the ability to move around(very slowly), and eat trees to regenerate their HP. Usually it is used for repairing the Ancients via the Eat Tree function or moving them to a new location, as they are too slow and too valuable to be used as combat units on a regular basis.
 * The Undead faction's Obsidian Statues in normal form act as mobile healers and mana restorers, but can later be switched into Destroyer Form, turning them into powerful flying anti-caster units. Unlike all the aforementioned dual-mode units, however, Destroyer Form is a permanent change and a Destroyer cannot be turned back into an Obsidian Statue.
 * The neutral Pandaren Brewmaster hero's ultimate ability allows it to temporarily split into three different units with distinct abilities and spells.
 * Warhammer 40000 Dawn of War: Most units have both melee and ranged attacks, and switch between both when ordered. However, they go into melee if attacked by a melee unit, leading to problems with Crippling Overspecialization ranged units.
 * The Tau XV88 Broadside Battlesuits will fire their anti-infantry smart missile systems while mobile, but can be entrenched to bring their twin-linked, anti-vehicle railguns to bear.
 * The Imperial Guard Heavy Weapons team has laughable damage when mobile, but entrenches itself to become a long-ranged machine gun/laser cannon.
 * Long-range artillery units in many games often have a "packed" and "unpacked" mode that allows the unit to move or fire, respectively.
 * Trebuchets in Age of Empires II need to be manually packed to move and unpacked to fire, and take several seconds to do so. Thankfully, they'll do the latter automatically when they move into range of the target.
 * Artillery units in Age of Empires III are capable of moving while in their firing mode, but do so at about a third normal speed. With the exception of the Iroquois Light Cannon, anyway.
 * Artillery units in Rise of Nations switch modes more and more quickly as you move forward through the Tech Tree -- initially it takes quite a few seconds with the catapult, which must be assembled and armed, whereas it takes only a split second with the rocket artillery while the missile launcher is tilted upwards.
 * Nod Artillery and the GDI Juggernaut in Command & Conquer: Tiberian Sun also follow this model.
 * The Civil War Generals games from Sierra had most units with two formations, typically one for movement and one for combat. Some units (the HQ and artillery units) were immobile in their non-movement formation.
 * World in Conflict: All units have 2 different secondary abilities, and/or types of ammo. Jeeps can repair other vehicles and change to heavier ammo, infantry can speed up, APC can fire anti-tank rocket etc.
 * Inazuma Eleven 2 has a few characters that can change modes, namely Fubuki and Kogure.
 * Super Robot Wars has multi-form mecha like Getter Robo, Zeta Gundam, or Aquarion. Most of these will sacrifce power and defence for speed. Usually one form will be MUCH more helpful than the others, but some games will make the differant forms more balanced.
 * Emperor: Battle for Dune: House Ordos' heaviest tank the Kobra has two modes one mobile and one stationary.
 * In Stronghold 2, knights and lords can be mounted on horses, turning them from the slowest to the fastest regular units in the game.
 * League of Legends gives us Nidalee a huntress that can with her ultimate ability switch form to a cougar to get a new set of abilities. She of course can switch back.
 * In addition we have Xerath, a mage shooting lightning who can root himself into the ground to get ridiculous range for his attacks.
 * Also there is Udyr who changes with wichever of his four animal aspects he activates making him a quadra mode unit.

Other Examples:

 * Non-video game example (but exists in videogame adaptions anyway), Future GPX Cyber Formula has a Mid-Season Upgrade machine of the protagonist able to change between three modes of the machine settings during a race: Curcuit mode for balanced adjust, Aero mode for speed favored, and Rally mode for offroad racing. The sequels remove offroad tracks, so Super Asurada is modified into having only first two modes. In addition, at the end series, every single team but one use dual mode cars to race.