I don't think journal entries for thousands of parts, for 15 years of comps is a viable solution, seems like it would be faster to edit each inventory item individually which allows the option for prior transaction.  BUT - Since this does work on item by item edits, IT SHOULD BE ALSO BE POSSIBLE for multiple edits also.  Pease provided the steps for multiple edits -  to get to that same pop up box, that allows past transaction option that is available with the item by item edit.