You can't use the fact that united let you book a ticket using an invalid promotion (which you knew to beinvalid but you tried it anyway) to get out of your obligation to pay for the ticket. You were lucky that UA even issued the ticket, but you can't get both the benefit of the discount (which you got) and the ability to invalidate the transaction. The invalid promition would potentially allow UA to invalidate the ticket, because it works against them, but because the error was in your favor, you don't have really have any grounds to do the same thing.

If it's a nonrefundable ticket, it will probably be treated as any other nonrefundable ticket, which on the new UA (old CO method) means you have to pay the cancel/change fee first, and then you can book a new ticket using the full value of the old ticket.