This can be in a book, game, film, show, whatever. Characters that when they changed, you found them less likable/entertaining.
For me, I'd have to say Ted Mosby from How I Met Your Mother. I still love the show to death, but Ted is a little irritating.
He's still Barney's foil (less so, I guess as they're really starting to want the same things) but I guess I preferred him when he was fulfilling that role by being down to earth and sardonic, whereas Barney was/is cheery, absurd and womanizing. I find him distinctly less entertaining as an ex-hipster/nerd, rather effeminate individual. It's also slightly jarring that his future narrator persona seems to have more in common with the original Ted than how he's become.