Call me naive, but don't they squawk a bit when you cancel the data plan and their spiffy new phone? Don't they want the subsidized phone back? Or are you allowed to keep the phone because you keep the 2 year contract? Would this work in my case - you said in your case both of your lines were up for renewal - in my case, the line where I want the new phone to end up is not up for renewal.
Generally they only want the phone back if you cancel within the first 14 days. Count on paying the first month's bill, activation fees, taxes, and ETF.
Still a good deal though.