Really need an answer sometime tonight on this

If someone could be so kind to provide some insight PLEASE
Okay, so, I wanted to confirm this theory that has been discussed and double check something because of upcoming questions about Christmas items being directly toward me this holiday season.
My intention is to purchase the Nexus, but, my fiance wants to give me a gift card to Amazon (which will almost completely cover the purchase price under contract of the phone). Here is the catch... I have a current contract and have a year left.
I've heard you can purchase a new smartphone with a new line of service. Get the data plan for it (account is grandfathered in for unlimited), and then move a dumbphone onto the exisiting line that I have, and remove all services from it.. essentially paying an extra $10/mo until the contract is up in a year for that line.
Is this possible, and, if so, can I make the purchase through Amazon when the Nexus becomes available?
Thanks for any advice!