You should be able to just go to the invoice and enter a new vendor number in the invoice. It will be greyed out, but you can still click it and type in it.

A popup will ask if you want to void the invoice and create a copy, click yes.

Then reapply the payment.

I've never done a cc transfer before, only checks/auto-draws/etc, so I'm not sure if that will cause an issue

As you know, Sage 100 doesn't allow editing posted transaction data.  When it comes to this kind of fix I ask two questions:

• What do you want to see in transaction history?
• What would an auditor want to see in transaction history?

With those answers, the "how" becomes clearer.