Daniel Radcliffe wakes up as Harry Potter. When he gets on the Hogwarts express it quickly comes out that Rupert Grint and Emma Watson have also found themselves in Ron and Hermione respectively.
They don't really look like themselves (Book appearances: Harry has green eyes for one) but they still recognize one another.
Looks like they're going to have to reprise their childhood roles, except this time things are already different. This time they're real, and in no way are they going to follow the "script" of canon.