Hi,
Already created PR and PO forget about ... since they are already created using existing MRP.
If you want to make MRP run Monthly basis, the date of MRP run is important. Ofcourse, since you maintained VB, system consider to create procurement proposals based on Reorder point and Safety stocks maintained in material master. I don't see any further consequences with this changes.
Fix ROP and Sstock at the start of month based on requirement and run MRP on specified dates. System created proposals accordingly.
Regards,