I have a question. I have in my purchase history, parallel version 15. I do not have that currently installed on my newest mac. Do I need to already have an older copy of parallels installed in order for the upgrade to work? Or do I need to purchase the full version since I can't install my old permanently licensed version 15 on my current computer? I tried to reinstall the legacy version of parallels, but that didn't work, so I'm not sure if I should purchase the upgrade or the full version.