In order to change the account on a single transaction, there are two routes:
If the transaction has already been invoiced, raise a credit and re-invoice.
If the transaction has not been invoiced:
Click Edit on the transaction.
Click the Address tab.
Click in the Switch Address box and select the site address under the new account.
If the site address does not exist, it will need creating.
Click Save.
By moving the transaction to an address under the new account, it will pick up the new account details on save.