Say you have 2 lines. Contract 1, and Contract 2.
Contract 1: Droid Charge with 18 months left on contract with the smartphone data plan.
Contract 2: You want the Gnex at the 2 year contract price.
If you upgrade line 2, in order to get the contract price, you have to sign a 2 year contract WITH the data plan. You can't take the data plan off by switching to a dumb phone since you got the Gnex @ the 2 year contract price. Sorry but I'm pretty sure that won't work and Verizon already though about it.
Your best option might be to sell your charge for ~$200 ish and buy the Gnex for ~$500 ish off contract. That way you don't have to open a new line and its still the same as the 2 year contract price (assuming $300) after selling your old phone. The prices may differ though but its still better.
If I am not mistaken, the 2-year contract is on the line, not the data plan.

