Replacement Flat Character

"I'm a teamster compared to you!"

—Frasier (to the former Trope Namer, Niles), Frasier

When a geeky/stilted/figure-of-fun character steadily becomes more cool, relaxed and well-rounded, this character is brought in to fill the vacuum and emphasize the original's new-found coolness.

Compare Suspiciously Similar Substitute, where the original character leaves before being replaced by a similar one; Over and Under the Top, where the dynamic exists to begin with; and Cousin Oliver, when a new character is brought in to fill the role of the cute child after the previous one grew up.

Examples of Replacement Flat Character include:

Anime & Manga

In Pokémon, May was brought in as a brand new starting trainer, as main character Ash was becoming a bit more mature and experienced with two regions and the Orange Islands under his belt. This would later be repeated when May and her Suspiciously Similar Substitute Dawn meet in the same Pokémon contest in Sinnoh. However, May hasn't completely changed...

Comic Books

Archie Comics has Cheryl Blossom, who was introduced as a Ms. Fanservice-type, but also as a contrast to Veronica, who had undergone enough character development that there was some need for a new version of her. Cheryl is bitcher, richer and skankier than Veronica was.

Films -- Animation

In Toy Story 2, Buzz was given one of these in the form of a newly unpackaged Buzz Lightyear toy. Like The first Buzz, he had no idea he was a toy. This only served to remind our Buzz Prime how annoying and stuck up he'd acted right out of the box. "Tell me I wasn't this deluded..."

Literature

In Warrior Cats, Darkstripe served as the Butt Monkey until he died at the end of the first arc. When his spirit returned to seek vengeance along with the other villains in the fourth arc, Darkstripe had managed to grow stronger because of all the abuse he had suffered and he actually posed a credible threat. Because of this, another villain called Snowtuft was introduced to be the Butt Monkey, and got pushed around and beaten up by the other characters in all but one of his appearances.

Live-Action TV

On Buffy the Vampire Slayer, Watcher Rupert Giles (Anthony Stewart Head) became steadily more relaxed, funny, confident, modern and so on, so in time, Wesley Wyndham-Price (Alexis Denisof) was brought in to fill the role of wet, panicky, geeky, trapped in the past, obsessed with manners and decorum, and having no sense of humour. Interestingly, Wesley then underwent a similar transformation as he transitioned from Buffy to Angel, becoming a fully rounded character in time.
In the Stargate Verse:
- Archeologist Daniel Jackson (Michael Shanks) becomes steadily more physical and cool as Stargate SG-1 goes on (and the writers run out of allergy and geek jokes). During the episode "Meridian", he is rendered unavailable/thought to be dead and is temporarily replaced by an anthropologist, Jonas Quinn who is essentially the big geek Daniel used to be, and remains an ongoing (intermittent) figure of fun.
- In one episode Robert Rothman was a geekier archeologist when Daniel was temporarily unavailable.
- Dr. McKay starts out as a recurring SG-1 character who shows up whenever there's a need for a scientist to jerkily disagree with the main characters. On Stargate Atlantis, he is one of the main characters, so he gets some character development and Dr. Kavanagh is introduced to take over the "recurring jerk scientist" role.
Sawyer on Lost started out a Jerkass and developed into a Jerk with a Heart of Gold. In season 4, he exhibits little (if any) jerky behavior at all, but then Miles was introduced and became the replacement jerk. This is lampshaded even before it becomes blatant:

Miles: Where the hell did they go, tubby?
Hurley: Oh, awesome, the ship sent us another Sawyer.

The Mary Tyler Moore Show: When Mary's friend Rhoda got her own spinoff, she brought along her younger sister Brenda to take on the "less cool friend" role.
In Red Dwarf, Rimmer ends up being his own Replacement Flat Character, so to speak: in the TV series, the "original" Rimmer gradually becomes more fleshed out and—if not likeable -- understandable. He even shows the occasional moment of bravery, before leaving in series VII to become "the next Ace Rimmer". Then in series VIII he's resurrected by nanobots as his old, pre-hologram self, and is back to being "you as you used to be" as a disgusted Lister puts it.
- Meanwhile in the book (and less strongly in the episode "Me^2", the contrast is made between Rimmer and the copy of him that hasn't been "mellowed out" by Lister and co.
After the eponymous star of Chuck grew into a confident, competent and somewhat superpowered spy, his even-geekier best friend Morgan (who had been in the show since the beginning) was brought into the spy team, allowing him to be the same fish-out-of-water adventurer that Chuck used to be.
Frasier was introduced in Cheers as an uptight, prissy intellectual who was usually the butt of jokes. Over time, he developed and even had his days in the limelight; for example, the episode "Love Is a Lonely Snipe-Hunter"). Once he was given his own show, Frasier, and became the central character, he had to become more complex and well-rounded, eschewing one-dimensional prissyness for more of a straight man role. Cue the appearance of his brother Niles, who was everything Frasier used to be and more—geeky, neurotic, and the butt of (almost) every joke.

Web Animation

In Homestar Runner, Strong Sad has become much more strong and confident, losing a lot of his Wangst and occasionally getting his own back on Strong Bad. Therefore Coach Z and the King of Town have filled in as "pathetic losers" in his place.

Web Original

Gap: Tom was originally somewhat uptight and nerdy, but eventually became the relaxed character of today, and had his role filled by David.