Hi,
You can easily change it for all the products in bulk.
Create a new massaction on the "product" data via the menu System>Mass actions, and use an action "update the values" on the column product_sale_start with the value 0 and process the mass action. Note that you want to change the limit to 1500 in order to apply the action on all your products and not only the first 500 (or you can do it several times with different starts).
