From what I'm hearing, as long as you are going to another smartphone and not a feature-phone, you will keep your unlimited data plan. It applies both to 3G and 4G phones. Feature phones also had the option for the $29.99 unlimited data package, but it coded differently in Verizon's system than the $29.99 package for smartphones. That's why some customers were confused and upset when they got the $29.99 package for their flip phone hoping to be grandfathered and when they went to a smartphone, they had to get a new, tiered data package.
I do know that if you move from a 4G to a 3G phone, the LTE sim card in the 4G phone will be deactivated and never work again. When you move to another 4G phone again such as a Bionic, you need to get a new sim card. Its inconvenient but I hear corporate stores will give you a free sim card.
If you go from a 4G to another 4G phone, you can keep the same LTE sim card.