I'd think you'd only be able to take out the instantaneous deflection from the loads currently on the truss, the rest will be locked in deflection due to long term creep (i.e. occurred over the 32 year life of the structure), and not removable by even applying the load slowly.
Once you jack past the point of taking out the existing loading, then you are loading the trusses in the opposite direction which is potentially a bad thing (think elements designed as slender tension elements now in compression).
I don't wholley really agree with Rons approach depending on how much of the excessive sag deflection is due to long term creep and how much is instantaneous deflection. If there is no creep then I agree with what Ron is saying, but after 32 years 1" of that 1.5" deflection might be due to creep, and you're not going to recover this in my view by jacking unless you wanted to jack it over a very long time period (could take 32 years if you apply the same load in the reverse direction!).
The method being proposed is like imposing an elastic deflection of 1.5" on the current truss arrangement in the upwards direction to take out the instantaneous + creep components. Truss may be only able to take 0.5" instantaneous deflection before members fail elastically for example (a guess but demonstrating an effect).
Over the short term of this recovery loading I'd imagine there is minimal creep going on. So I doubt the truss could handle this negative loading condition, but at the very least it's something that needs to be assessed, and presumably the locked in loads in this condition need to be accounted for in the final arrangement after the addition of the strengthening and then release of the jacking and ultimately some sharing of the total locked in loading, normal original loading, etc.
If it were me i'd jack out the current loads or some proportion of them, then strengthen on the basis of the new and old sharing the loads based on the final arrangement. If there were still residual deflections, then I'd look at treating this by some other means like redoing ceiling battens or redoing flooring, etc.