DOCTOR: I could be a curator. I'd be great at curating. I'd be the Great Curator. I could retire and do that. I could retire and be the curator of this place.
CURATOR: You know, I really think you might.
(Yes, that is the current silver haired version of the fourth Doctor you just heard. There's Tom Baker, leaning on a walking stick.)
DOCTOR: I never forget a face.
CURATOR: I know you don't. And in years to come, you might find yourself revisiting a few. But just the old favourites, eh?
This bears little resemblance to regeneration as we know it. We've never known a Time Lord to regenerate to aged versions of previous incarnations, much less repeatedly.
Allowing that The Doctor believed, even tentatively, The Curator to be the genuine article, I can only speculate as to The Doctor's speculation on how he might become The Curator. Aside from giving Tom Baker a cameo, the point of the scene was to establish a future direction for The Doctor: finding Gallifrey. One might speculate from the remaining dialog, that The Curator is The Doctor, after having found and saved Gallifrey, and retiring.
How might The Time Lords thank The Doctor, at the end of his lives, for having achieved such a momentous outcome? The gift of being able to live out his retirement as his favorite versions of himself seems generous, within The Time Lords' power, and indicative of their perception of The Doctor as an irritant. "Gee, Doctor, it's really great that you won our war, saved our lives, and restored us from limbo, but it would be really great if you could just have a seat over there."
Bear in mind that The Doctor doesn't know much about Trenzalore yet: he doesn't know that his battles there will be connected to Gallifrey, nor that they are going to happen soon. The Doctor might have rationalized that his death at Trenzalore happened after saving Gallifrey and his time as The Curator.
