Can I Change the UOM on an Inventory Item?
If you can't edit the unit of measure (UOM) on an inventory item, the UOM is locked. It locks once either of these is true:
The product has a purchase order that isn't archived.
The product has been used on any quote, invoice, or purchase order.
When the UOM is locked, you can't change the product category, the rolled-good setting, or the unit-of-measure fields directly. Use the workaround for locked items below.
Other fields on an inventory product that has stock are also locked on purpose, not only the UOM. If you need to change one of them, the "Workaround for Locked Items" below works for those fields too. To learn why the fields lock and which product-level edits the Consider Base Price For Inventory Product For Quotes/Invoices setting allows, see "Why Inventory Product Cost and Price Aren't Editable in the Product List".
How to Edit UOM on an Unlocked Item
If the UOM on an inventory item isn't locked, you can edit it directly on the product.
Go to Inventory.
Find your product and open it.
Update the Unit of Measure field.
Click Save.
Workaround for Locked Items
If the UOM on an inventory item is locked, you can't edit it on the existing product. Instead, create a copy of the product with the correct UOM, move any stock over to it, and hide the original.
Duplicate the product. Find the item, click Duplicate, and give it a name that reflects the correct UOM.
Set the correct UOM on the duplicate and save.
If the original has stock, move it. Add a reduction adjustment on the original to bring it to zero, with a note like "Moved to new product – UOM fix." (See How to Add an Inventory Adjustment.) Then add the same stock to the new product with Add Existing Inventory, converted to the new unit. For example, if each carton covers 20 sq ft, 10 cartons × 20 sq ft = 200 sq ft.
Mark the original as invisible so it stops appearing in product searches. It stays on existing jobs for history.
Use the new product from now on.
Setting up UOM during import? See Inventory Import: Understanding Unit Of Measure (UOM).