Customer number change and National Accounts

Client has a request to change the customer id but the customers belong to a national account.  Reason for the change is that the customers are changing affiliations and client built in national account identifiers in the customer number.  Most of the customers have balances owing on their account as well.  The client wishes to keep transaction history.

My process if the customers have a zero balance would be to change the national account first and then use the customer number change to change the customer id.  I don't think this would be an issue.

I can use this for customers that have a balance knowing that anything outstanding will be under the old national account but how would this affect payment of old invoices.  I assume that it is still O/S for the customer no matter what the NA.  Would it cause an issue if they post receipts by NA as opposed to the member account?