Hi @Alison65W
When you change your contract in any way as you have done with a different plan with increased data, to bring you in line with the new tariff the following months bill will be charged as pro ratered, this will mean you will be charged part of a month for the previous tariff and part of a month for the change. There will also be a credit for line rental paid in advance prior to the change.
When you get the December bill, you will need to check the dates before and after the change. If you are still not sure, live chat will be able to go through the bill with you.
As this is the same as upgrading, this link will give an explanation: Pro Rata Billing/First bill after upgrading
