My new roof has a few sagging spots
I had a similar issue on a house (built in 1973) that had 3/8" plywood for roof sheathing. Over the years it tended to bow downwards, leaving an odd look from the curb. To solve it, I cut multiple 2x4 stretchers that fit between the trusses and boosted the plywood even.

You didn't say if it was a truss roof or rafters, but the solution should work the same either way. You could install the 2x4 stretchers at the joint of the sheathing and it should level the roof out. I will say that unless you have good spacing in your attic, it will be a practice of straddling the trusses and getting in to tight spaces.