OK found this on the web... To keep unlimited.
FIRST you need have two phones one dumb and one smart. And the dumb phone is up for an upgrade. Upgrade it to the new smart phone you want. You will have to add a 2G plan on that phone but that doesn't change your smart phone unlimited plan on your other smart phone. Later move the new upgraded phone to your old smart phone line and move your old smart phone to that line. Just swapping phones.
Next change that old orig dumb phone line that now has your old smart phone on it back to the old dumb phone and cancel the 2 G plan on it. Now you are back to your orig plan one dumb phone and one smart phone on unlimited. All you did was swap phones.
I ran this across a Tier 2 Verizon online service tech and she said go for it should work.
Would like to hear more from or others who have gone to Best Buy...
Goldenthree: Who posted....
I would recommend going to a Best Buy. I too have a grandfathered unlimited plan, and the Verizon store was going to switch me to a new 2gb plan if I upgraded. I went to Best Buy, they upgraded me (at a lower cost than vzw directly), and kept my plan unchanged.