Characters who have been written differently after the show found its pace.
Spoilers for all shows ever.
Not in terms of a character naturally evolving, but writers having an idea for a character's direction early on and eventually going, "Nah, doesn't work as well."
I considered Phillip from The Americans for this, since he seemed more forceful in earlier episodes. But I think the change could be attributed to his ongoing depression and losing faith in his cause.
- Always Sunny — Dennis, Mac and Charlie were all originally manchildren trying to run a bar, do stupid shit and get laid. Dennis became a psychopath, Mac turned gay and Charlie became more childlike. That early storyline with Charlie's ex bringing a kid into his life probably wouldn't have happened in later seasons, or at least not the same way.
- Mad Men — Pete Campbell originally seemed he could be a future threat to Don, based on his ambition in the pilot and a few remarks made by others (Roger, I think). He showed no real promise of this later.
- Community — Abed seemed less robotic in early episodes.