You can only swap the ESN after the rebate has processed (30 days), it's easier to go to Best Buy and have them activate it on the line you want from the beginning. There's no wait period to switch because there's no MIR to deal with.
I work for Best Buy Mobile , I was implying this exactly they way you said it. Get a phone from Best Buy and there are no mail in rebates, phone can be put on immediately.