No dropping of VAC. You can swap with another pilot, at least in theory—it’s akin to the PSB for trips, minus the drops, but without any mechanism in place to do so. It’s on you to find another pilot in category with the exact same number of VAC days (including any SUPP added and/or reductions) who wants to trade.

The move-ups are automated in iCrew: if the week(s) you want are available, you’ll get it.