Switching back to automatic numbering for PO after using manual numbering

Is there a way to automatically revert the PO numbering to automatic after someone selects manual?  We usually use automatic numbering but sometimes a buyer will need to enter an alpha-numeric number so they'll change the numbering to manual. If another buyer enters a PO before the setting is reverted back to automatic, they'll get manual numbering. We'd like to have it default to automatic numbering. Is this possible?