My wife is in the same boat: Silver on CO and nothing on NW. We just got back from a trip with 2 legs each way. Wait for the first legs to do the automatic upgrade and then change at the airport. The next day, call in and change back to the CO number. Wait for the upgrade and change at gate on the return. We had no problem at both airports with the agents changing the FF#, but wait until you get your boarding passes and then ask for the change in the computer.

Most times it works, but sometimes the computer gets screwed up and you get some mileage on both airlines. Just request missing mileage and they will correct.

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