FM2017 can probably open FM2.0 files. So far, every new release can open the binaries (.fm) of any older release. Expect dialogs for anything older than FM6, and unavoidable Unicode considerations prior to FM8. FM9 should be trivial.
Older releases can open newer ones too, if MIF is used as the interchange format. New features will not be present on the older platform and may be lost from the document. Don't round'trip unless only old features are used, and don't try to round'trip across the FM7.2/8 Unicode boundary.