Jason confirmed at GB Fan Fest that the original core cast (inc Weaver & Murray) are all on board.
No word on how much screen time though.
He actually made an explicit point NOT
to confirm they were in the movie at that time. Weaver had just given the Parade interview saying everyone was back and Breznican was grilling him on it. Reitman ultimately conceded they had read the script, and that they formed a brain trust to make sure the film honored the original movie, but would not admit they would be in it.
While it is arguably one of the worst-kept Afterlife
secrets at this point, this seems to be -- for now -- one of the things he isn't confirming.