Thanks for reaching out to the Community, wendy-cann,
I'd be happy to show you on how to change an existing customer to a CIS customer.
To start, let's turn on CIS feature. Here's how:
Once done, to change the existing customer to a CIS, follow these steps:
That's it! For more information, here's a great resource that you can check: CIS in QuickBooks.
If there's anything else you need help with in setting up your customers, please let me know. I'd be happy to help.