its available right now, or it’s not. Space in PZ needs to be available for an upgrade award to clear. If it’s not, they take the miles/co-pay, and put you on the list in priority sequence. If space opens, it clears people on the list in order (with disclaimers like, if there are 2 on your reservation waiting, and only one seat available, they will skip your party and upgrade the next 1 pax party).

United isn’t ‘selling’ you an upgrade if you don’t receive it. At time of request, you always have to pay the miles/co-pay upfront, so they know you can pay for it. If you don’t clear, everything is refunded (without fees).

if it does clear, you’ve got a confirmed seat in the cabin, and they can’t ‘revoke’ it. If you aren’t sitting in that cabin, they are required to pay downgrade comp. the only reason that would happen is if they can’t provide it, ie, plane gets downgraded to one with fewer seats and they are now oversold. Is it possible? Sure. Is it likely? Probably not.