depends on the sagging.

if the joists are all even and level to one another and the sagging is only in the plywood itself, you can fill between the joists to level, then clad on top of it.

if the actual joists are uneven to each other, this complicated things a bit.

this can be taken care of by more jacking and blocking or tear apart the whole subfloor system and scab or replace joists to create the level and evenness between joists, then install new subflooring.

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I think it will be complicated. I am guessing it is the joist. Thanks though!