https://ca.news.yahoo.com/disney-faces-star-wars-size-dilemma-over-loss-235015748--finance.html
Re-casting her seems completely out of the question to me, and I don’t think the fans will be able to just accept her suddenly being gone from ep. IX through some excuse in the opening crawl. I have a feeling that she’ll have too big a part in ep. VIII for that to make sense. Lucasfilm/Johnson have roughly 10 months or so to “solve” this problem, so they might have to give her some heroic death scene, most likely (partially) off-screen like ferris suggested. It could work narrative-wise as Rey has already lost a father-figure in Han, so doing something similar for ep. VIII could work.
There’s obviously no ideal solution when something like this happens, but I trust Jonson and Lucasfim will find a solution that will be respectful to both Fisher and her character, while also feeling more-or-less natural to the plot of the future films.