If a deposit is associated with an invoice for a subcustomer, is there an easy way to simply "transfer" the funds to another child or the parent account? From what I'm reading, you can't change the customer on the deposit once it's set. In fact, I can see it can't be edited and assume there's no way to override this for parent. We do NOT want top-level payments only.

you can use a Journal to make it available for the other Customer