I'd be glad to provide info with changing the PO number to another number.
At this time, QuickBooks Online doesn't offer a feature for changing the Purchase Order generated to start another one. I am going to leave feedback to our Product Development Team for them to review and consider making this an option soon.
In the meantime, you can change a Purchase Order manually in QuickBooks Online.
Begin by ensuring the setting is enabled from the Gear icon > Account and Settings > Expenses tab > Purchase Orders > check-mark Custom transaction numbers > Hit Done.
Navigate to the Purchase Order you'd like to update and open it.
Edit the value under PO no. Once you're finished, select Save & Close.