Mario going from 2D to 3D for me. Now, I know that the 2D games are still around, but the 3D games became the "main" games, and I also really prefer them to the 2D ones (Super Mario World is still great though!).

How about Romance of the Three Kingdoms (strategy game) turned into the Dynasty Warriors fighting game turned into the Dynasty Warriors 2+ hack and slash game?

I would consider FFXV a (sub?) genre change vs the previous FFs.

Star Control 2 I'd say is an adventure game vs. Star Control 1 which was a strategy game (although they shared the same combat).

Many games when jumping from 2d to 3d, tho not necessarily changing genres.
FF going from turn based to action based, tho not considering the hideous FFXIII
Alien going from suspense/horror to action in Aliens

Fallout 3 did alright for itself after switching to a FPP RPG.

GTA has seen moderate success after switching perspectives, though it still remained the same type of game.

RPGs changing from turn-based to action combat, it just makes sense. Glad there are still dedicated Turn-based/Tactical RPGs though

Jak & Daxter going from wacky platformer to darker 3rd person adventure/action/shooter style, easily the best game in the series.

going from 2d to 3d its not genre change.

I liked starfox adventure.

Mario changes genre all the time, started platforming, then go karting, then board games , party games a bunch of sports.

So far Im against everything said in this thread. Not a game has been a genre change, just an advancement in tech. 2d to 3d, should not count. Linear to open world either. also changing camera positions, first or third person view is not a genre. And for the one who said turn base rpgs going action rpgs, this is the only one i think is closest. Still rpg is the base game.