I think:
Spoiler
The Doctor will bring back Gallifrey and undo the Time War, effectively meaning that he and the Ninth Doctor never existed in the first place. He'll be a temporal anomaly long enough to say goodbye to his companions, then vanish. Then the Eighth Doctor will regenerate into Matt Smith (whether we actually see Paul McGann or not).
It wouldn't be as if the last four seasons were erased from the canon, they'd just be moved over. And his companions and the Time Lords would still remember what happened, even if the Doctor himself and most of the people he interacted with don't.
It wouldn't be as if the last four seasons were erased from the canon, they'd just be moved over. And his companions and the Time Lords would still remember what happened, even if the Doctor himself and most of the people he interacted with don't.