You can use a Flexi for Windows too. The licences are not OS-based. If you have a dongle for ANY Flexi 8.5/Flexi 8.6, in addition to the licence code you need a download of the Flexi Mac install files only. As far as I remember, Flexi 8.5v2 Build 1655 is the latest build. If you need a download, just ask, I have the install package of 8.5v2 Build 1655 available.
Because Flexi uses new dongles with version 8 or later, nowadays there is no chance to upgrade older versions. And, be aware of that Flexi Mac runs up to Mac OS X 10.7 only. Some installations were successful on Mac OS X 10.8 too, but some modules of the software are failing.