Can verizon iPhone 4 be used as a prepaid verizon phone?

I have an iPhone 4 that was on my contract. I'm getting a new phone soon so I'm selling my phone to a friend but he wants to use it as a prepaid phone on verizon. It seems to me it wouldn't make a difference whether or not it was originally prepaid or not, but I as wondering if anyone knew the answer. Can a old contract phone be used as a prepaid phone?

I believe the iPhone should be unlocked for it to work prepaid.