I travel on YUPs regularly and 100% of the time when there have been flight cancellations, I have been reaccommodated in Y with no refund provided. While you are theoretically at the head of the line for upgrades (or at least ahead of those awaiting sticker upgrades), in practice it is up to the gate agent and the prevailing opinion seems to be that these are coach fares, where upgrades are processed after all confirmed passengers are upgraded. If the $100 represents a small fraction of the fare, and you want to maximize the likelihood that you actually sit in business class, then I would pay it. It is very aggravating to pay a premium over a regular coach fare and then end up sitting back in coach when things go wrong.