Welcome to the Community, andy-bate.
Let me share some information about changing the expense status in QuickBooks Online.
The status for the expense transaction isn't changeable. Expense transaction is a cash transaction and the status will automatically set to Paid.
I'll be sure to take note of this product feature opportunities for future enhancements. Once we receive some progress or enhancement about this feature, I'll make sure to inform you by updating this thread.
Additionally, I'd encourage you to visit our QuickBooks Blog, so you'll be updated with our latest news and updates including product improvements. Just go to https://quickbooks.intuit.com/uk/blog/small-business/.
For additional help, you can also reach out to our QuickBooks Online Support.
Should you need more help with expenses in QuickBooks Online, don't hesitate to let me know. I'll be here to help you out.