What do the Virgin Mobile grandfathering rates do?

I've been a VM customer for almost a year and is looking to buy the triumph. I've heard that older users will have their rates "grandfathered" but I don't really understand the concept. Instead of paying the 35 per month, someone with an account before the 20th pays 25 per month instead? Like say in December 2011 i'd still pay 25 instead of the 35? In which case, how does that work if I pay with the pre paid cards you'd buy in person?