I agree with the above posters - you're out-of-luck for changing your ticket mid-stream, but you should be able to use an SSWU if you've got one. Try selecting "upgrade" if it's there during OLCI (if it's available in FRA), failing that show your SSWU when you check in at the airport.
But it sounds like you might be out of SSWUs and wanted to pay to u/g so you could use SWUs? In that case I think you should have upgraded your ticket when you added the stopover.