Automating ID Numbers Before Handing Over Database

I've got a database, that I'm trying to update for my sister in law.I've got 2 tables, Items and Customers.As I've been working with the id's they have used so far, these aren't currently automated.

for customers, they use their number in the auctions, to place bids etc, so I can't change them.But once I hand over the database, I want to automate the generate of Customer ID numbers from that point on, but not override the history.With the items, what I have done, as an item can be relisted up to 3 times, I created a LogID auto number which is also my key number.

Before I hand it over, I need to do a big update there with them, to pull in the latest customer and item information.But after that, I'd like the new items added, to be given a new automated number, but the logid still needs to be generated automatically as well, as the login allows no duplicates, whereas the itemID does allow duplicates.

Its the item ID that is used to generate an invoice and show how many times its been listed to pick up correct number of fee charges.Customers can be both buyers and sellers, and I had to allocate 2 different fields, and the buyerID just replicates the sellerId number. So the Seller number is the main number on the table, I didn't want 2 tables of buyers and sellers separately, its one table.