W/O knowing the reason the floors are sagging, that should stop you from doing anything. If the joists are 'bowed' and in good condition, they may simply need to be sistered and leveled. But if the sagging is due to settling problems, rot problems, or the like, the solution may be more dramatic.
Did you have the home inspected? And what did the inspector say about the flooring? Competent, ASHI-approved inspectors are able to adequately diagnose and estimate the fixes if they're able to 'do their inspection' w/o having to be disruptive to the structure of the house. If you haven't had it inspected, now's a good time. (Get one that's ASHI approved too.)
My best to ya and hope this helps. (If you have answers for me/us, I/we're listening ...)