Since you have a 1-1/2 story with finished rooms on the second floor, I'm going to assume that you have knee walls connecting the floor joists to the rafters. My guess is you should be able to see some separation somewhere in the framing on those walls since the floor sagged, unless the rafters sagged along with them. At any rate that is all the more reason to level up the structure before reroofing. And all the more reason to jack this thing up VERY slowly so things have a chance to settle without tearing joints apart. I would bet that the knee walls are going to want to push up on the rafters as you jack things up.