Maybe I'm wrong but if you can't support the upgrade, you still have your confirmed economy seat as long as you got a seating assignment prior to getting to the airport.

If you used a paper certificate to support your upgrade, the upgrade process is not finalized until you get to the airport and they lift your paper cert. If you do not have the paper cert, you can always take your assigned coach seat.

If you used e-certs, then you needed to have them in your bank prior to confirming the upgrade prior to travel for CR-1s and SWUs (and be issued your upgraded searing assignement), or at check-in (either online or in person) for 500-milers.

Am I missing something?