Claudio,
actually you need to create a new Class (Security) ID and Issues/ Purchases every time you want to see what is the behavior of different Amortization procedures, as there is no update once PMP is recorded in the deal. I know this requires a bit more efforts, though this is the way to do it.
try it yourself, otherwise I could try to check this most probably around the w/e on some IDES.
/Renatas