Hopefully not this as the start of the opening crawl for 9:
General Leia Organa has died!
If IX - as it's been plotted at this point based on where Johnson has taken things in VIII - hinges on a Leia/Kylo Ren confrontation, then yeah... they'll have to radically rethink Kylo Ren's character arc. I can't see Leia's absence affecting the arcs of Rey or Finn in a dramatic fashion, so those elements of the storyline are probably going to remain mostly as intended. But if Leia was instrumental in concluding or furthering Kylo Ren's journey, he's probably going to get shafted, which is kind of a bummer because I think he's the most interesting and complex of the new characters. You can't just shift that story responsibility onto some other character (unless it's maybe Luke, I guess, as he'll be the only character left from the original trilogy with a familial link to Kylo).
I wouldn't want to be in the position of having to figure this stuff out, but I'm of the opinion that VIII should be left alone as much as possible (unless it's supposed to end with Leia in some kind of cliffhanger situation), and sort things out in IX.
Potential silver lining - maybe this means Rian Johnson will stick around to help re-fashion IX's screenplay.
It would have worked significantly better as a 13 episode, high budget stand-alone HBO/Netflix series.
The whole book is so rooted in 80s pop culture. I don't see how you can recreate War Games with Matthew Broderick for one key sequence. Or how you can have Ultraman, Mechagodzilla, every sci-fi movie and TV show ship all in one huge battle and make it feel like a credible, exciting climax on film.
I feel like Spielberg is going to rework the tone and quite a lot of the bigger and more important scenes to actually make a passable movie.
If there was more for Leia to do in EPISODE IX, I feel like they should see it through to the end and let her go off into the sunset. If you want to honor Carrie Fisher, complete her work, don't make it a bridge to nowhere.
If they go back with reshoots and contrive a way to kill Leia (good luck without at least some CGI), how do you do it without being distasteful? How do you still honor Carrie Fisher's work and the character?