Hi @RDRD,

I'll share insight on your query about your customer's credit memos. 

It's already a given that QuickBooks Online (QBO) would no longer automatically apply these open credits to invoices as long as the feature is turned off. Please keep in mind though, as my colleague @Kristine Mae stated above, open credits are present whenever you receive payment. If you don't wish to apply these credits to the invoice payment, ensure to remove the check on its checkbox. These open credits are located under the Credits section.

You can refer to the screenshot below:

You can take a look at this article as well: How can I transfer a credit from one customer to another? It has 2 sets of steps on how you can transfer credit from one customer, then apply it to another. 

Don't hesitate the post a comment below if you need further assistance. I'll be sure to get back to you.