Typically I've found that Sprint CS will not swap the upgrade dates on line - I tried about half dozen times. However you can order a phone for one lines upgrade and activate on another. If you do this you probably need to order from Sprint or wirefly/letstalk/amazon/etc will slap you with termination fee if you don't put the phone on the correct line.
Someone at a non-corporate sprint store said if my wife and i didn't mind switching phones for a few days, he would be able to do something. But then personally I'd feel obligated to order from him and at the time, the best deals for what I wanted were online non-sprint, so i passed.