Hey moorbern,
It's great that you've been able to identify this! Addressing errors like this goes a long way in helping keep consistent books and taking full advantage of QuickBooks Online's powerful features. The fastest way to update the supplier on your expenses is to edit them individually. This ensures no errors are cause in the reporting, and helps the system categorize them correctly. To view expenses affecting a specific account, check out the awesome Transaction List by Account report.
Hope this helps!