How Can I Change a Client's Preferred Payment Type?

The Preferred Payment Method is located on the client's Contact page. If you edit this page, it's located on the bottom right. 

 

You can change this to check (or CC, Cash, etc) and that will, going forward, be the payment type that appears on the Completion page. 

 

Changing this has no affect on the CC numbers stored in Cloud, so you can change this without worry! When a CC is marked as "Default", it means it's the first CC that should be used on the account (should there be multiple numbers). So you want to keep whichever is the primary CC marked as "default", but the contact page is where you'll set the Preferred METHOD of Payment.