I've not watched most of the X-Men films after the first couple, but clearly they've been making them long enough that they've needed to re-cast people. As far as I know, that was done by shifting to the "First Class" series/timeline, with younger versions of the characters. So there's two different people playing Xavier, Cyclops, etc. (and yet still only one Wolverine...)
But I can't recall if they ever needed to re-cast any characters within any/either of those timeline/series of films. I suspect Beast was recast in the original run, cause I don't think Kelsey Grammer did more than one film, but I could very well be mistaken.
They've done so in the main Marvel films (Rhodey being the obvious example) but I'm not sure of any in the X-Men films. Anyone?