I think the original pick should stand. People are going to be annoyed either way if they end up getting burned, but I think it would be significantly more frustrating to have my primary voided, especially if he ends up beating the LL and the backup loses. I think the possibility of an opponent's withdrawal is simply another element of chance that we have to either accept or take into consideration when making our picks. I feel like 9/10, I'd want to stick to with my primary pick anyway. What would happen in the event that the backup had already lost? If the original pick were voided, you'd be eliminated (or there'd need to be another contingency plan to support this contingency plan).
Just my two cents.