Are they saying you cant upgrade from a PC version to a MAC version? I use a PC. I have installed an upgrade version on a new system that didn't have PS installed and only had to supply the previous versions registration number with the upgrade version registration number. I wonder if Adobe thought you wanted to install the upgrade on both the MAC and PC. In that case you would need both versions. Seems to me if your upgrading you should be able to upgrade no matter what platform.