In fact, one agent said someone else might grab my FC seat if I tried to get the fare difference.
Of course, that could also happen if someone BOUGHT your F seat, since presumably the only way this could happen is if you're repricing a discounted fare that isn't instantly upgradeable at time of purchase, and you're waiting for the 72 hour/48 hour MVPG/MVP free upgrade.
That's the thing- it's not a gimme that you'll score an upgrade to F anyway if you are buying out of the bargain bucket, so taking the cash and taking your chances on going down the queue seems reasonable to me- plus if you are MVPG, you automatically trump the MVPs no matter what.