All you have to do is to use all your AS miles first and then stop getting miles on AS. I have a similar situation with NW an CO. I have more miles on NW but i like CO's route better since i'm alot closer to IAH then say MEM,MSP or DTW, so when i DO get reward travel...i'll just use CO, but i have no plans to move these things around...it's just too much headache.
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Albert