In this bit of the movie, it could be possible to dub in a Threepio line from TCW or other material which implies he now remembers the prequels as well. He talks for quite a while, and as long as he still mentions ‘Mistress Rey’ somehow it would be clear he remembers everything now and hasn’t regressed. More radically, you could perhaps find a way to move this scene later in the film, or even mask out the whole background or something, and use it for a ‘R2 and C3PO telling the story of Star Wars’ moment or something similar.