They will at least need her likeness to appear as a force ghost- Obiwan, Yoda, Vader and now Leia.
Carrie Fisher did a self biography show in 2010, the video of which was recently aired again after her death. In that show, she lamented that George Lucas owned all rights to her likeness, and she could do nothing about it as it was in the contracts. She joked about seeing her likeness everywhere in commercial products, and that as she aged the inevitable comparisons were thrown in her face. She accepted the changes in her appearance as she aged, but some fans were downright mean regarding her no longer appearing young and thin.
It could be that Disney only owned her likeness on existing material while Carrie was alive, and not on new material upon her death.