Question about terminating a line on my family plan

[brownpm85]

So I have decided that I am going to get a Moto Droid. I have a family plan with 2 9530 Storms on the plan. Here is what I have figured out will be my lowest cost method over all. I have decided that I will add a line on my family plan and get the droid for that line, I have also decided that that I don't want to put a dumbphone on that line and pay the $10 as it will cost me a total of $240 over the 2 year period. I started my plan in august so my ETF is still $175 which is cheaper than the $240 it would cost to keep the added line. My main question is can I add a line and then switch the droid over to my main number and swith the Storm1 to the new line then cancel the Storm1 line for the $175. But someone said that if I do that the storm then becomes connected to the new line and my ETF will be $350. I will just switch numbers if that is the case, the hassle of changing my number is better than having to pay the huge etf. So I guess my main question is, is the contract tied to the phone number or the phone that is on the line?


Edit: This is a Verizon Wireless plan