Somehow I doubt that what you ask for is possible. Perhaps I am missing the full extent of your question, but here goes anyway. Within Zen Cart, there is quite a heavy dependence on valid id numbers. For example, orders are linked to customers via customer id. So hypothetically, if you were to change a customer's id number, you'd have to change all its occurrences in the three customer-related tables, the customers_basket table, the customers_basket_attributes table, the order table, and probably a bunch of others as well. Not easily done.
Or perhaps I'm missing something which might become clearer if you explain your situation in a bit more detail.
Regards.
Apsona
http://apsona.com