How to manage Aging of Items?

Dear All Experts,

We are using Inventory, PO and OM Modules since 3 years, now we analyze that our items codes are not correct and needs to maintain new items codes against them. So now like previous item code is AABB123, so for same item now three different item codes needs to be generated like AA123, BB123, and AB123.

Now first we need to take all items close from the transactions and take out all inventory on hand balances.

After that we are maintaining new items codes, now if we are going to enter new items codes in to on hand inventory than there would be no aging of items present. So how could we manage items aging and its costs for newly created item? We are using Average Costing Methods.