Like finkface, I am a bit confused. (But that's nothing new!)

My understanding of your situation is, you buy a ticket having selected the MVP Gold Upgrade fare type. You find, after having completed the transaction, that you cannot upgrade into the F cabin as you expected you would be able to.

I made a dummy JNU-LAX rez and selected the MVP Gold Upgrade fare type. That brought up many available flights, many of which had the gold U at the right edge of the Flight Information box. Hovering the cursor over any old gold U brought up a small dialog box that said "Flight Available for MVP Gold Upgrade". If I had made a choice of flights that had these gold U's and, once the purchasing part was done, I found that I was not able to consummate the upgrade into F - I would be pi$$ed. That said, I have never had that experience before. I just always made sure to confirm that the upgrade was available with the cursor-hovering maneuver prior to completing the purchase.

I also noticed that some of the flights in my dummy rez had greyed-out U's with slashes through them. The dialog box for those U's said "Instant Upgrade Unavailable". That may be what's happening with you, but I can't imagine you would buy a ticket with a slashed U and then expect to get an instant upgrade after the transaction was complete.

Can you clarify if this was the process you've been following?