Virgin Mobile occasionally changes their phones, devices, plans, and prices. When Virgin Mobile changed prices on plans in the past they "grandfathered" existing customers on those plans and let them keep their existing plans as long as they didn't change plans, change phones, the plan was discontinued, or let their accounts expire. Only new customers paid the new price.
See these FAQs for examples of Virgin Mobile USA's policy:
answered 1 year, 8 months ago