Domestic mileage upgrades from any fare (with no co-pay for elites) are a great benefit on UA. If you book far enough ahead, you can often get one before the UDU windows open. Such things do not exist in AC's program.

I don't know if they will poach from *A, but you can try to get a status match/challenge if you are able to fly 10K UA+CO miles in 90 days:

http://www.united.com/statusmatch

I know other AC travelers are asking for it.

If you don't status match, you can use some of your *S (silver) perks on UA by showing your card. If necessary, enter your AC number for free bags then switch to your UA number and reprint boarding card.