I am CP on US Air and plan to buy a ticket to Hawaii with a mixture of US Air and United flights from CLT for me and my wife (Silver Preferred). Am I correct that the only way to upgrade a coach ticket is to buy the 500 mile upgrades from United and then try to upgrade at the day of the flight? has anyone had success doing this?

The other option is to say the heck with the US mile earning and just buy a ticket on AA and upgrade with miles. I would prefer the US option if possible for the mileage earning / status reasons. Thanks for the help in advance. ^