GR/IR Conversion issues

Hi Experts...

We are going through conversions of GR/IR accounts i.e. assigning new accounts for GR/IR Clearing. Can anybody tell me what are the steps to perform this activity efficiently. FYI - some PO's are still open that is not yet invoiced, how system will take care of this if GR/IR account will be changed.

Thanks & Regards,

MV