Some news:
JJ will come back to direct Episode IX. The film also moved the release date to December 20, 2019 and Disney moved the Aladdin live action movie up to the May 2019 release date. This sets up an interesting confrontation, as Warner Brothers just announced it had finally inked Patty Jenkins to direct Wonder Woman 2 and set its release date to December 13, 2019. Will they keep it there now that it will run into Episode IX in its second week, or figure the holiday box office is big enough to support both blockbusters?
Speculation: Trevorrow's last Episode IX related tweet was August 22. Disney fired him around this date, offered the film to Rian Johnson and were turned down. Disney then entered a bidding war for Jenkins which WB only won by offering her $9 million and points on the backend which could be worth millions more. Disney then turned to JJ and the deal was done quickly. Just speculation and we may never know for sure, but given the timeline that seems to fit.