I think you might be confused.

First, assuming you still want the upgrade...just stay on the waitlist. If it clears, they'll take 15k from your account and you get to ride up front. If it doesn't clear, steerage for you.

The only time you have to pay the difference from your "discount fare class" is if your upgrade is CONFIRMED and you want to downgrade.

When you confirm an upgrade, you're actually removed from your discount economy class (S, T, W, V, Q, and I think "L") and placed into the upgraded class (NF or NC). If you want to leave NF or NC for some reason and go back to coach, you'd only need to pay a fare difference if your original discount economy booking class were no longer available.