What we do know is that whatever form Weaver takes in Avatar 2 will be evolving over time. A few years ago she said that her part is a little bit different in each sequel and that she'll "transform somewhat" between them. However, it's possible her character has been transforming even within the script writing process. Even more recently, Weaver has said she's not actually playing the same character in the sequels as she did in the original. So, maybe Grace Augustine is dead after all, even though Sigourney Weaver will be back. It's hard to say for sure at this point.