Can I change a customer to a supplier. The customers have expenses attached to them?

Currently, changing a customer to a supplier isn't an option in QuickBooks Online (QBO), @ron-j-barton. You'll have to inactivate the customer's name and recreate it as a supplier. I'm here to guide you how.

In QBO, there's a proper way of changing a customer to a supplier. You'll need to first perform the inactivation process from the Customers page. This applies to those customers that already have transactions associated with them. Before doing so, please make sure to secure a copy of all the expense transactions for this specific customer. This way, you can add them to his vendor profile.  Once ready, Here's how:

1. Go to the Sales menu.
2. Select the Customers tab.
3. Locate the name of the customer.
4. Under the Action column, select the Make inactive option.

After that, recreate them as a supplier from the Expenses menu. Here's how:

1. Go to the Expenses menu.
2. Select the New supplier button.
3. Enter all the supplier's necessary details.
4. Click Save.

To effectively manage your suppliers and expenses through QBO, I'm adding this article for your reference: Suppliers and Expenses. It contains topics about handling supplier credits, deleting an expense, and creating purchase orders to name a few.

Please let me know if you have other concerns. I'm just around to help. Take care always.