[WorldMaker]

Right, the radical change is showing the Second Foundation getting built (and it having interesting reasons, in part due to individual mistakes, why it is getting built very slowly in time) not that the Second Foundation exists.

It's definitely an interesting storytelling change, and probably for the better. Second Foundation in the books is very much a seat of the pants deus ex machina where Asimov seemed to write himself into a clever puzzle with no easy answer and cleverly solved it "at the last minute" with a retcon, then sort of took another trilogy and a half to complete the retcon and deal with the consequences.