This would be great news. Where did you hear this?
Souter has expressed in interviews that he has no intention of dying in office - he wants to retire and do some writing.
John Paul Stevens is 88, but relatively healthy and has indicated that they will have to carry his body out of the courtroom. He won't admit it out loud, but he is gunning for two records. If he lives to 91 in office he will be the oldest justice to ever serve, and if he lives to 92 in office he will have served longer than any justice in history.
Common knowledge here in DC.
I hear Justice Ginsburg is not in the best of health.