Hi Joseph,
This is not good practice to change the decimals places for the currency to which documents are already posted. you also would have got warning message when you changed the decimal places.. It would affect all the posted documents in the system.. It would be a hectic job to bring back to normal.
I had similar experience but in my case, there was NO Payment taken place.So, deleted all the MIRO and reversed MIGO and manually changed the PO value and posted it with correct value.
you had better to create new currency with required decimal places ( for eg: USD - 2 decimals and make USDN - 3 decimals as such) and use it whenever required.
With Regards
S.Siva