I have repaired quite a few hatches
over the years and have found it easier to leave it mounted from the stand point of cutting it out to re-bedding the new one. I just finished replacing the windows on a 36 Jaguar and the principle is the same. Cut it out, remove old adhesive, prime.mask off new plastic, mask off hatch
or window, caulk and clamp then do a final cosmetic caulk. This is a little simplistic but the steps are the same. The primer comes in a small can and is available from automotive glass shops and is applied by brush. Be it a 10" hatch
or 6' long windows it works and be sure to read the directions on the adhesives. 5 minutes of masking will save 5 hours of clean up.