Change Unit of Measure without Changing Previous Records

[kyart]

We are currently using Peachtree Premium Accounting for Distribution 2009. For quite a while we have bought and sold items by the dozen. Now, we need to sell these items by the pair. I know how to go in and change the item so we can buy and sell in different qtys. My problem is that when I change an item, all of our old purchase orders/sales orders/invoices change as well. For example, we bought 5 dozen of an item for $16.00/dz, for a total of $80.00. Now that I have changed the stocking u/m (and the purchase multi-pack), it looks like we bought 5 pair at $16.00/pr. Of course, this affects our inventory and, more importantly, our profit margins.

Is there any way to change the stocking u/m without affecting the old records?
Thank you!