How to handle retrospective price and rate changes?

We have a requirement here where the price changes and rate changes happen retrospectively. For e.g. if periodic billing of one month has been done till June 2009; there could be a price change which can happen from march 2009. In this case, a debit or credit has to be passed to the customer for the differential charges. Here we are using back billing to take care of this requirement. But the problem is with MRU, Portion and Rate category change. SAP doesnt allow you to change an MRU, portion or Rate category without doing a final billing. Is back billing being used for the right purpose here? Or should we look at other alternatives like adjustment reversal ( We understand that this will create a huge number of reversal documents in the system).

Thanks and Regards.