The OP might be able to get an exception if he is a high tier elite with UA as a courtesy. I'm assuming that the ticket was purchased more than 24 hours ago if UA has this policy rather than allowing holds. (One or the other must apply to a USA domestic ticket.) Otherwise the options are as people have suggested above, although abandoning the tickets and taking another carrier should still give a UA credit for the fare minus the $150 change fee toward another UA ticket within a year.