It's my understanding that there shouldn't be any problem accessing a file hosted on FMS12 with an FMP13 client, and that FMP13 features (popovers, etc.) will work fine — as long as there are no server-related FMP13 features used (e.g. Perfrom Script on Server).
On my dev setup, when I set a file to be "Minimum version allowed to open this file: [13.0]" and upload it to FMS12, I get a "Warning: File uploaded but could not be opened." The log shows Error 106 "Database 'x' could not be opened. Database version incompatible; try opening in FileMaker Pro."
Is this expected behavior? If so, can anyone foresee any issues keeping the minimum version at 12 & using an old-fashioned on-open script to check that client is 13.0 or above? (In this situation, there's no risk of anyone with FMP12A overriding the script step)