Hello @SeanGerard,
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.
Lastly, here are articles you can read for ideas about your inventory management: Inventory and Projects for QuickBooks Desktop.
If there's anything else that I can help you with, let me know in the comments. I'll be here to lend a hand.
