I wouldn't be surprised at all to see a regeneration on Saturday. The episode title could well refer to the day that he starts calling himself the Doctor again.
Even if there's a regeneration from Hurt to Eccleston, there's the possibility of revisiting the War Doctor. It's not as though his life were brief! He may have had the longest life of any of the incarnations.
Yeah, really! Given that Eleven went two hundred years without visibly increasing in age, the War Doctor looks like he's been around for about 8,000 years!
3000 or 4000 ought to do it.
Alternatively, it could be explained away by this incarnation being part human, just like Eight.