How to adjust prepayment

Hi Guru's,

              I have a Issue where I have already applied prepayment to a standard invoice in year 2012. but due to some internal reasons,I have to un-apply the prepayment, which was allready applied to that Standard Invoice in year 2012. but the main problem is, that year 2012 is already closed & all of a sudden if i un-apply the prepayment to that invoice in the current date, it will affect in year 2013 prepayment balances of 2013 which i dont want.

             So please advise, how to adjust the prepayment which is already applied to a standard Invoice with out affecting year 2012 or 2013 prepayment balances.