Excel 2007

I have a sheet called SALES RECORDS and another one called PRODUCT LOOKUP. I use a table in PRODUCT LOOKUP to return product costs when I enter a sale in SALES RECORDS. It's simple, but it works fine. Except.

Today I changed the cost of a product, and that cost changed was reflected on ALL previous sales. Is there a way to change the data in product costs so that the costs for previously entered sales will not change?

TIA,
Cane