I haven't been able to find the answer to the following question anywhere: When is the last possible chance to upgrade an eligible fare using miles on a *A carrier? I know you can't wait list. But can you check with your mileage program (in my case UA) while sitting at the gate of the *A airline? Or is there a cutoff point after which no upgrade is possible (other than upsell at checkin). My specific question refers to upgrading on B class AC using MP miles, but I would be interested if there is a general policy.