Will be cutting the vent stack(PVC) and making changes. When I install new PVC parts, can I slide the vent stack a little ways up thru the roof boot so I can make my PVC connections with PVC cement?
It depends on the boot. If it's lead, maybe not. If it's a rubber boot, possibly. If it's cold and brittle or covered with roofing cement, it might tear. Instead of moving it, cut the pipe and use a repair coupling (no ridge in the center so you can slide it over one end of the pipe completely).
I have a rubber boot that does not have anything cementing it to the PVC pipe, and I have slid my pipe up and down a couple of times while making some changes after first lubricating the boot and pipe with some dish soap.