Headspace won't change by much because that would require the shoulder to wear or distort and, mostly, it doesn't. The threads will wear in to a certain extent and, generally speaking, after a half dozen removal and re-installs, the barrel will go past register at the same torque or even a little less torque. If the contact faces of the threads match up real well, the barrel will register for more cycles. Acme thread and square thread are a little better than vee threads in this regard. I have a lot of rifle on which I switch barrel quite often. On some, whether or not the barrel registers precisely doesn't really matter but on those with extractor cuts and/or iron sights, it's more critical. As the threads loosen, turning the barrel to the same index will require less torque and the headspace will increase but not by much; maybe 1 or 2 thou at the most. Unless the shoulder wear or distorts significantly, for some reason, headspace will not decrease at the same torque setting; regardless of where the barrel indexes.
