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No retune will be needed at the higher voltage. I tried it out with my motor at 48v. It was tuned, and then I tested it at 96v, and cut the Kp and Ki values in half and sure enough, I got the exact same convergence curve. I think your tuning idea of half/double rather than linear increase sounds much better. It's probably a lot faster to do it that way.
By the way, It's OK if the PI loop doesn't converge to zero with Kp = xxx, and Ki = 0. You just don't want it going negative. It should level off to some fixed value close to zero though. Then, slowly increment Ki 1 at a time to get it to converge to zero. And I think a 1 or 2% overshoot wouldn't be a big deal. Also, There's going to be some noise, so if it's going +27, -35, +13, -12, -40, +31, that wouldn't count as overshoot. It's just noise.
We are concerned about going from positive to negative, where it is more negative than say, -60.
-Paul
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