Hi,
As per Standard SAP the G/L accounts are triggered from the automatic account determination, if the PO is having the a/c assignment then the G/L would come from the PO. So it is not possible to change in the MIGO screen as per SAP Standard practice. But what is the business requirement to change in the MIGO, the right G/L is to be maintained in the PO.
BR,
Suresh P