I have been in this situation...

You have a couple of alternatives, none of them pretty. You can keep trying and see if R (upgradable) fares become available. If the flight were 3 months out this might work, but I'll bet it's next week.

You can hang in and hope for the Silver UG. I am doing a transcon MR this weekend to make Gold and thought about using miles for the sure thing. But then decided to risk it. More exciting that way.

Or you can *shudder* pay for F. You can run it on nwa.com and see how much this will cost you - probably a lot.

Not a lot of help, I know.