We changed non inventory items to inventory items. We discovered, the hard way ,that selecting the box that does not affect previous periods doesn't work when COGS are involved. Would the sales tax be affected too? We have manually updated all of the previous periods that were affected by this change, but now are worried about the sales tax implications. The items in question are taxable.
Changing the type of the item won't affect the sales tax code used. As long as you did not make any changes to the sales tax settings of the said items.
With this, your items will still be taxable even after changing them from non-inventory to inventory items. You can verify this by reviewing any of your sales reports with the said items and checking out the TAX column of the said transaction.