Originally Posted by
WineCountryUA
The app was then offering a non-refundable / non-transferable upgrade fee and those may not book into a paid fare class,
In general, GG BUYUP / change is not going to be a compelling replacement for SDC for domestic routes. Due to differential-based pricing, the currently available J/F fare is going to consist of the fare difference (between what OP paid originally and the lowest-available Y now) plus the add-on for the fare class differential. In other words, it’s “whatever it would cost to SDC, plus the upgrade fee.”
It never hurts to check, of course, but I’d be surprised if it worked out frequently.