How do I convert a Member/Customer to a Vendor and not lose past transactions?

Hello there, @treasurer93.

Converting a customer to a vendor is unavailable. You'll want to edit the customer's name to a different one. This is to avoid getting a duplicate error.

Here are the steps:

1. Go to Sales > Customers.
2. Select the customer's name, and click the Edit button.
3. In the Customer Information window, modify the customer's name.
4. Click Save.

Once done, create the vendor by following these steps:

1. On the left panel, click Expenses > Vendors.
2. Click the New vendor button located in the upper right.
3. In the Vendor Information window, add the vendor details.
4. Click Save.

To know how to transfer the transactions made going to the vendor account, you need to delete that transaction and create them under the vendor.

Let me show you how to a transaction under the vendor:

1. Choose Expenses at the left navigation pane.
2. Select Vendors.
3. Choose the vendor you need to create the transaction.
4. In the Action dropdown, choose the transaction you need to create. 

You can also refer to this article on how to enter and pay bills in QuickBooks Online: Enter and pay bills.

After recording the transactions, you can make the customer inactive. You can follow the steps here: Make a customer inactive and then active. 

Please let me know if you have any additional questions. I'm just a post away to help you some more.