I have my gp DM no. in all my US reservations so that I confirm at the window for the unlimited upgrades. Most of the time I seem to do OK.

I was wondering if once I confirm for the upgrade, can I switch out the number for my UA number so that I ride the F on US but accrue at UA?

Or will my switching the numbers undo the upgrade?

and if it wont undo the upgrade when is the best time to do it? - I was thinking after I get my F boarding pass I might go to another GA and try to do the switch. Will they know I was DMU'd or will they just be able to switch the numbers and not affect the upgrade at all?

For the record... I am not jumping ship- It's just that I reached my 50k prefmiles and I think rather than try to make the run for cp, I would like to accrue some UA miles for UA upgrades etc.

I remain optimistic regarding US's future.

Thanks for the replies.

It will undo the upgrade. So, either fly Coach or book A-class fares.

This is answered in the US/UA Upgrade thread posted as a sticky at the top of the forum