How do you change an existing customer to a cis customer?

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:

1. Click the Gear icon at the top, then choose Account and Settings.
2. Click Advanced in the left navigation panel.
3. Go to the Construction Industry Scheme (CIS) section, then click the pencil icon.
4. Enter the necessary information.
5. Click Save.
6. Click Done.

Once done, to change the existing customer to a CIS, follow these steps:

1. Click Sales in the left navigation panel. 
2. Go to Customers tab, then locate the customer.
3. Click the name of the customer, then click the Edit button at the upper right hand. 
4. In the Customer information window, put a check mark in the box Is CIS contractor.
5. Go to Tax info tab, then enter their UTR number.
6. Click Save.

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.