My guess is Verizon was offering you an early upgrade providing you upgrade to a 4G Smartphone. These phones carry higher pricetags and need higher priced plans. This translates into more profit for Verizon which is why they do it. They won't let you you upgrade to a basic phone because they don't make as much profit on those phones. Just wait until your normal upgrade date and then I'm sure you can upgrade to a basic phone.
That could be true about profit but more importantly the more people they have using 4G the opportunity they have to get more 4g coverage areas. That's why they are pushing all 4G devices. I know a couple of people they were talked out of the Iphone for this reason.