I think there are some movesets that are too canonized to make drastic adjustments to, particularly characters where Smash has taken some liberties to flesh them out and kind of make them its own. Main ones that come to mind are Fox, Ness, Captain Falcon, Marth. In fact Marth's moveset is one of the most influential in the whole game, having roots in four separate characters. Dancing Blade will certainly stick around.
But then you have characters who are either more fluid, like Mario and Link, or clearly welcome further adjustments like Ganondorf. Obviously they don't feel quite so attached to Ganon's moveset if they threw out all his smash attacks, for example. But they also don't want to completely abandon what's been established, so they ease out of it. But then some characters like Sheik, Zelda and King Dedede saw world changing new moves added in their second or third games - Burst Grenade / Bouncing Fish, Phantom Slash, Gordo Toss respectively. Imagining Dedede without Gordos is impossible for me now.
Some changes, like the one I just mentioned, make a character that much more appealing and complete without betraying those who already enjoyed them and inviting even more players in on the fun. I guess it just comes case by case.