Monster Upgrade

A Sub-Trope of Mon series where monsters gain power through evolved forms or other means. The most well-known method is through a kind of "evolution" into a different and stronger form either through growth or a catalyst. Other means involves fusion with another monster to make a new mon, alternating into a different form, or a Super Form.

Sometimes happens with the aid of a Mon Machine.

Anime and Manga

 * Digivolution from Digimon is achieved by the digital monsters download additional data to get stronger. Once enough data is downloaded, Digivolution is triggered. The six stages of Digivolution are Fresh, In-Training, Rookie, Champion, Ultimate, and Mega. The event happens by age, and enough data from the environment has been taken. When a Digimon forms a bond with a human, then Digivolving occurs faster. The use of a Digivice can make Digivolution instantaneous compared to other methods. The downside is that they will devolve to their prior form after a certain time.
 * Armor Digivolution can be achieved through when a Digimon absorbs energy from a Digi-Egg(or Digimental). The power of an Armor Digimon is usually comparable to that of a Champion.
 * DNA Digivolution is a method of fusion two or more Digimon to make a higher-level one.
 * Mode Change is where a Digimon becomes a different version of their current species gaining more power than their normal form.
 * DigiFuse is similar to DNA Digivolution but involves the main Digimon fuse one or more into new body-parts or weapons for a new form. This method needs a Fusion Loader by a human to achieve different forms.
 * Guardian Ghosts from Shaman King can evolve to a higher class of spirit if their Shaman has more Mana than their Reiyoku. An alternate method for Human Class spirits is by existing for 500 years or shedding their physical attachment.
 * Organoids from Zoids can give Zoids more power by fusing with their core. Another ability is to evolve into an upgraded form which is their primary function.
 * Legendz from Legendz have alternate forms for upgraded attributes and also more powerful attacks.
 * Bad Transformation and Super Transformation from Flint the Time Detective where Time Shifter transforms naturally or artificially. Bad transformation or Kon form occurs by being the Petra Stam or Uglinator's Skull Mark which gains a larger form and more evil personality. Super Transformation or Master form occurs naturally by accessing their hidden inner power but for a limited time. The form can even be achieved while in Kon form.

Fan Works

 * In the RWBY/The Gamer crossover fic The Games We Play, Jaune Arc eventually learns that he can combine the elementals he summons to create higher-level, more powerful elementals.

Live-Action TV

 * Contracted Mirror Monsters from Kamen Rider Ryuki and Kamen Rider Dragon Knight can evolve if their Rider uses their Survive Card. Kamen Rider Ouja can fuse his three monsters by using his own Unite Vent card which then allows him to use his strongest attack.

Tabletop Games

 * Yu-Gi-Oh! has the violet-colored Fusion Monster cards, which normally comes into play by using a Spell Card called "Polymerization" to fuse at least two specific monsters into a stronger amalgamation of them.
 * Union Monsters work on a similar principle, except that the "host" usually gets a stat boost instead of creating a new monster. And it's worth mentioning the three that go with X Cannon Head and V Tiger Jet can also be used to create several combinations of Fusion Monsters with varying effects in addition to the stats that they would normally get when united.

Video Games

 * Evolution in Pokémon is where a Pokémon changes physically while also having different stats, types, and Abilities. The phenomenon can occur due to experience from battle, friendship, exposure to items, weather, and trading between trainers.
 * Forms or Formes are variations of a Pokémon based on factors of weather, exposure to items, current environment, a condition in battle, and usage of certain items.
 * Mega Evolution transforms a Pokémon into a more powerful form that is achieved through the use of a strong bond, a Key Stone, and a Mega Stone.
 * Dynamaxing has a Pokémon increase in size and also changing their moves in battle. There is also Gigantamaxing where certain pokemon gains a special form and a G-Max Move.
 * Demon Fusion in Shin Megami Tensei is where one or more demons are fused to make a new one. Different factors influence the result from the demons themselves to the phase of the moon.
 * Demon guides in Devil Children can absorb the power of their Demon Child as their strength grows and evolves.
 * Evolution introduced in Shin Megami Tensei III: Nocturne has demons that evolve that are unobtainable through fusion.
 * Personas from the Persona franchise can evolve based on the growth of their user's personality.
 * Style Change from Megaman Battle Network 2 temporarily changes a NetNavi's element and weaponry. NetNavi's style of fighting determines the Style while the element is random.
 * Double Soul is an ability to gain added powers of an allied NetNavi while in combat through sacrificing a Battle Chip. The power occurs whenever two NetNavis' souls commune.
 * Cross System is where a NetNavi gains the traits of a Link Navi and gain their power and attributes of their form like Double Soul. The difference is how it can initiate anytime in battle rather than the sacrifice of a Battle Chip
 * In Medabots, instead of the titular robot evolving, its the Medal that evolves, which means that it gains a new Medaforce attack.
 * Enhancement from Robopon happens when a Robopon gets to a certain level.
 * Evolution in Spectrobes is in three stages of Child, Adult, and Evolved. The Child stage being used to detect Spectrobe fossils while Adult and Evolved are used for combat. Evolution is achieved through Minerals feed, how many battles they won, and what level they are.
 * Super Revival from Fossil Fighters: Champions can make either boost a Vivosaur's stats or evolve them into a more powerful form. This is done through silver or gold fossils found in Wonderous Fossil Rocks or Miraculous Fossil Rocks, respectively.
 * Natural Evolution, Fusion Evolution, and Lab Evolution for Denjuu in Telefang. Natural Evolution is done by leveling up a Denjuu to evolve to the next stage. Fusion Evolution has a Denjuu fuse with an item to achieve another stage while not being able to evolve any further. Lab Evolution uses the DNA of certain naturally evolved Denjuu and combining it to a higher stage.
 * Combining from Monster Rancher series is where one breed of Monster as a Primary is fused by another breed with as a Sub-type. An example would be a Pixie fusing with a Golem gains rock-like attributes on their horns, wings, and their fur being replaced with stone as well.

Western Animation

 * Powerbonding in Huntik is when a Titan has a rate of synchronization with a Seeker that gains more power. There are also capable of communicating telepathically with their Seeker. A Seeker may gain additional powers granted by a powerbonded Titan.
 * In Battleclaw, Jinlins can be fused into a more powerful creature that uses their attributes and elements.
 * Megamorphing in Slugterra is when a slug hits 200mph and can handle the higher speed. The slug can still megamorph even when shot out as 100mph.
 * Kaijudo: Rise of the Duel Masters has two means of evolution either temporarily through a spell or permanently through a serum.
 * Monsuno have Fusion Monsuno that 3 Monsunos were caught in the explosion of the Fusion Core Reactor which gave the ability to fuse with a specific Monsuno temporarily.