What major plot aspects are changed for Ascendant in regards to Rey and Palpatine?
Palpatine’s plan is streamlined to rely less on an unspoken sense that his spiritual heir needs to be (or is in some way better to be) a biological descendant. He appears now to desire Rey dead as the last Jedi, and makes an apparently credible threat to Kylo Ren that he will possess him if he disobeys. When Kylo is taken off the board, he directs Pryde to prepare to lure Rey whom he tries to manipulate upon arrival, using the less than effective playbook he used for Luke back in the day.
When Rey has a vision of herself on the Sith throne, she has something that can continue to unsettle her even after the redemption of Ben Solo, whether she’s aware of it or not. And Kylo’s pitch is now for the two to join forces and destroy Palpatine rather than usurp him. Rather similar now to Dooku’s offer to Obi-Wan in Ep2. Kylo appears to desire Rey’s alignment with himself, similar to TLJ, more than to use her as a convenient political ally. She has more now to be tempted by in his offer, as this is not the specific thing that she had feared from her vision.
Leia and Luke were not aware of Rey’s Palpatine lineage. It does not make sense that Luke would have known during TLJ, and I don’t know how Leia would possibly have, although the EU is obligated to straighten that out by some means at some point. Luke knows what Rey knows by the time they meet in this film, and calls her hangup by asking, “So what?”
The dagger is no longer explicitly Rey’s parent’s murder weapon. This reduces the power or coincidence, albeit one that isn’t too much of a stretch. It can now be something Ochi happened to have, and it’s spiritual direction rather than technical allows one to suppose it could be ancient, directing the user to the Wayfinder wherever it might be. (Although perhaps one must be in the same ballpark, as 3PO still needs to get them to the right general spot.) One could imagine, if Ascendant V2 were the released film, hearing that in the visual dictionary the dagger was actually what was used to kill her parents. And this change helps the emphasis the film puts on Rey sensing darkness with the dagger lend itself to helping with the change about speaking to the user with spiritual whispers rather than a technical compass instrument.