Hello there, wrmcm. 

Let's make sure that we have an open invoice to which we'll transfer the funds and apply to. It'll stay as credit or sales receipt to the new customer where the payment will be applied.

We can go back to the new customer's transaction list if there are open invoices. Then, we can open the payment again and reassign it to the new customer. When we go back to the transaction list of the old customer where the payment was applied, there'll be an open invoice. 

Let me use an example. We have an invoice for customer A and misapplied a payment that was already deposited that needs to be transferred to an open invoice of customer B. We can open the payment transaction of customer A and then change the customer into customer B. Read for prompts so we don't mess up your books. 

You can refer to this article to know more about receiving customer payments. 

I'm here if you need more help. You take care always!