I'm here to yield some details about editing old splits in QuickBooks Online (QBO).
If you won't make changes in Check Date, Total Amount, and Bank Account, you can directly open the split transaction and edit it. So you dont need to create check or payment in the system.
But if you make changes to that, I'd suggest communicating with an accountant. This way, they can guide you through the whole process of modifying the transaction to avoid any discrepancies.