When you update the service and non-inventory item, you have the option to select the Also update this account in historical transactions checkbox. For inventory items, on the other hand, it'll only affect future transactions. To update the historical transactions, you'll have to edit each transaction and toggle the item again manually.
For more details in tracking your inventory, you can check out this link: Set up and track your inventory in QuickBooks Online.
Let me know if you have other questions. Have a good one!
