I have an existing supplier who has just become a customer. QB will not let me create a new customer as the record already exists as a supplier

You encounter that error if the name already exists in the program, GO2. Let me help you get around with it.

First, you'll have to change the supplier's display name. Here's how:

1. Click Expenses.
2. Go to the Suppliers tab.
3. Select the supplier to open their profile, then click Edit.
4. Change the Display name as to "last, first". Or, you can also add any letter or Middle name.
5. Click Save.

Next, create the supplier as a customer. 

You can also refer to this article for more information: Error: The name supplied already exists. Another customer, supplier, or employee is already using this name.

If there's anything else that we can do for you, don't hesitate to drop a comment below. Take care!