The Star Wars Official Blog:
The role of Leia Organa will once again be played by Carrie Fisher, using previously unreleased footage shot for Star Wars: The Force Awakens. “We desperately loved Carrie Fisher,” says Abrams. “Finding a truly satisfying conclusion to the Skywalker saga without her eluded us. We were never going to recast, or use a CG character. With the support and blessing from her daughter, Billie, we have found a way to honor Carrie’s legacy and role as Leia in Episode IX by using unseen footage we shot together in Episode VII.
Along with Mark Hamill returning (Ghost Luke‽) and Billy Dee Williams reprising his role of Lando Calrissian, this little nugget was the most interesting. Fans, or at least I, had been wondering how they would handle the character of Princess Leia.
Now we have our answer.