Since the Master was used as a weapon in the time war odd are The Doctor was too.
The timelords probably forced 8 to regenerate into something that could end the war(Hurt)
Hurt does the job and uses "The moment"
Using one of Moffat's favorite story trick "It happened but didn't happen also"
Hurt is erased and McGann returns but can still remember it happening(Moffat story trick #3453)
McGann runs away but is injured and regenerates into Eccelston
You know, I bet that's what will happen. McGain regens into Hurt, does the horrible, runs away, returns to London faces off against Ten and Eleven, regens into Nine (everybody seems to be saying that Nine isn't in the 50th too many times, it's starting to feel like a cover up) and it leads into "Rose". Credits.