Reshoots on all these tentpole movies have become so ubiquitous that I'm not even sure if they still mean a movie is in trouble, the way they used to. Probably not, since they now seem to be budgeted and scheduled right along with the main production.
X-Men: Days of Future Past is the latest big budget blockbuster heading back to set to do more filming, according to the Calgary Herald via Coming Soon. Though no one knows which cast members exactly will be involved in the additional shoot, we do know it's to for additional scenes rather than reshooting any existing ones, and CS did dig up this tweet from Daniel Cudmore, who plays Colossus:
Back in Canada and its freakin freezing…and a tad grey.. Other then that it's great to be back
— daniel cudmore (@danielcudmore) November 15, 2013
Since Colossus is part of modern day X-Men, we can probably assume other modern day mutants will be needed back on set as well, so Ellen Page, Anna Paquin, Shawn Ashmore, Halle Berry and Patrick Stewart are likely to be involved. And with at least Jennifer Lawrence (aka Mystique circa 1970s) not back in Canada, logically this sounds like it could just be additional modern day footage through and through.