if your grandfatherd in to an unlimited data plan as long as you don't switch to a tiered data plan with VZW you can keep your unlimited data even when upgrading your phone wether to another iPhone or to a droid. i do believe switching to a 4G phone might be a different story though. the best thing to do is to check with an actual VZW store, not one in Best Buy or and Authorized Dealer as they don't always have the right answers
Verizon Rep: 3G Customers Can Upgrade to 4G after July 7th And Keep Unlimited Data
Grandfathered Unlimited Verizon Data Survives 3G-to-4G Transition
Details Of Verizon's New Tiered Data Plans Emerge
"The really good news is that you can keep your existing unlimited plan, EVEN when you upgrade your phone in the future. Unless you add a new line to your account, it shouldn't be going away anytime soon"
after that contract is up, or if you do another 2 years, that you'll need to choose an actual data plan that's not unlimited.
either that is not true....or thats pretty lame, i've been with verizon forever