I think I read who become Poe would have had the Millennium Falcon if Harrison wouldn’t come back and the Luke McGuffin would have been for another Jedi if Mark said no. I do remember Carrie being the first to agree to return. Mark and Harrison took more time to convince.
It just hit me that seeing the first teaser/trailer for The Last Jedi will be super emotional. Seeing Carrie Fisher as Princess Leia one last time. Bittersweet.