I adjust in down on my gun, max 1/4 turn. I take 2-3 dryfire shots between every turn. But It is probably more like 1/8 turn each time, as I stop when there seems to be more resistance. So far my reg seems to work just fine. If you are planing to adjust down allot (20-30bar), it is probably more convenient to do it the "old" way, and take the bottle off, and empty the plenum first, and then adjust lower than needed. Otherwise it takes a while, and allot of dryfire shots. If being in doubt you actually has the AMP reg or not, I would do it the old way, just in case. As far as I know the AMP reg came with the impact x. If I had a older generation impact I would not do it.
What you call creep, will also happen if you bring the gun in from colder temperature, to room temperature. I would not bother with changing the reg, as the huma will probably behave just the same.