If you have been following the news and images related to X-Men: Days Of Future Past you realize that we've seen images of Beast both as the mutant and as human form, played by Nicholas Hoult.
As the movie is set in alternate universes and timelines, perhaps that explained the reason for the switcheroo.
Or maybe it was some sort of tech developed by Hank McCoy to hide his Beast form.
Turns out neither is the case as Nicholas Hoult reveals the reason for seeing both versions of McCoy in the movie (via JoBlo).
What's happened up to this point is between the time of the last movie and this movie my character has created a serum which basically controls his mutation so his appearance is normal as long as he doesn’t get worked up. Any animal instinct or urges, that kind of brings him out. So yeah, he changes into Beast a few times throughout the story and they've done some great action sequences with him this time, particularly in the mansion flying around on these chandeliers and stuff.
We see mention of a serum, which seems similar to what Beast first created to cure both himself and Mystique, but then something went wrong leading to the furry blue guy we all know and love today.
It also sounds a little Hulk-like, no?
X-Men: Days of Future Past has a May 23, 2014 release date directed by Byran Singer starring Jennifer Lawrence, James McAvoy, Michael Fassbender, Nicholas Hoult, Anna Paquin, Ellen Page, Shawn Ashmore, Lucas Till, Hugh Jackman reprising as Wolverine, returning cast members Patrick Stewart and Ian McKellan, with Peter Dinklage as Bolivar Trask.