Here are the phones listed on Verizon's site that you can use on prepaid. I know 2 old phones that you can use on prepaid & that is the Env2 and Samsung U740. Both have been discontinued, but you can still get it from other places
There actually is a larger list of phones available for prepaid. i changed to my lg chocolate from the original prepaid phone i have. there should also be a posting elsewhere describing which brandand models you can use. Most of them are the older phones from two years ago.. hopethis helps.