O2 Tariff Change and Cancellation

I have changed my tariff with O2 as a prelude to cancelling my contract
early. They told me that they would change my allowance initially, but
that I would be billed at the higher rate, the following month the bill
would be adjusted for the over-payment.

Does anyone know whether if I want to cancel early they will use the
tariff I was billed for or the tariff I have been given the allowance
for? Would it be best to wait until my next bill when all the
adjustments will have happened?