Unlikely it will not be as much as 2 months. Your next bill will have the advance charges for the new bundle plus any excess between the change of service & your current billing date. But you will also receive a credit for the portion of your existing bundle charge already paid in advance beyond the service change date.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV+), Talk Unlim Telco, VIVID 100, Virgin PAYG Mobile