Hi Ben,
According to the licensing policy, the upgrade to Parallels Desktop 10 for Mac is only supported from the following versions: Parallels Desktop 8 for Mac, Parallels Desktop 8 for Mac Upgrade, Parallels Desktop 9 for Mac, Parallels Desktop 9 for Mac Upgrade, If your previous version key is for Parallels Desktop 7 or an earlier version,you need to purchase full version of Parallels Desktop 10.