My (F1) help says:
You can only enter a unit of measure code that has the value "1" in the Qty. per Unit of Measure field in the Item Units of Measure window.
You can set up different units of measure for purchases and sales in the Item Units of Measure window and you can set up translations for various language codes in the Unit of Measure Translations window
You cannot change the unit of measure code if there are open ledger entries for the item.