from memory the a original hood has a piping around the window and the hood material tucks under the piping then heat sealed, you need to get a pick tool and unhook the top material out of the piping then it can be unzipped refitted etc

I worked at a BMW dealership and replaced a few of Z3 rear window sections. They are not fun. The dealer stopped offering the rear window replacement and would only sell a whole top due to how often the replacements failed or did not fit. The biggest issue was the zippers. That was 8 years ago.

the only time I've had problems is miss matching oem with aftermarket.

No. Usually by the time the window has fogged up or cracked the stitching for the zipper is almost gone so when you go to zip in the new window the old zipper comes off an then you just have to replace the whole top anyways. Also when replacing those tops make sure you leave room for a possible sealing frame replacement (part that seals the rear tacking bar to the back end of the car body OEM only ~$3-400 from dealer usually a 50/50 chance it is cracked and needs to be replaced).