Did some digging. New ones are (modified, obviously) Boeing 747-8's. Current AF1 is a (modified) 747-200B.

747-8 has been in production since November 2005.

Boeing's website does not specify a completion date, just that the 200B's will be replaced by 8's.

Given their existing production line, I could really quite easily see an existing 747-8 getting the necessary modifications in significantly less time than ~3-4 years.

As an interesting side note, while I don't believe it is significant, the website features comparisons, with one of them being range - the 200B's can travel from DC to Tokyo, whilst the 8's can travel DC to Hong Kong.

You can read more about the plane here.

I do think that we may be on to something, though; all these pics with that in the forefront, and people were saying something about Dan Scavino posting pics of/with it?