Keeping multiples versions of the same film series from contradicting each other can be difficult. 2009's Star Trek, for example, had a very fine line it needed to cross. After all, if you revamp some of the most iconic characters in film and television history, you'd better not offend die-hard fans or alienate newcomers to the franchise.
Superhero pictures are particularly difficult since many comic characters have been around for decades and exist in different forms thanks to multiple writers, time travel and alternate dimensions. Nowhere is that more clear than in Marvel's X-Men universe. Consequently, there are two versions of mutant leader Professor Charles Xavier: Patrick Stewart's older teacher from Bryan Singer's original trilogy, and James McAvoy, who plays the character at a younger age in Matthew Vaughn's 2011 X-Men: First Class.
According to NME, Stewart has confirmed that despite his having handed the reigns of Professor X to McAvoy, he could be appearing in the next X-sequel. That's made easier for all involved because the next installment is to be called Days of Future Past. That classic storyline involves time travel and a plot to change history, so it makes sense to have people like Stewart and his fellow X-Men co-starts somehow involved.