Changing the UOM for an item

We created an item in August 2010.  We have not transacted the item in a Batch or Purchase Order. We cannot change the UOM. In our TEST instance which is still a clone as of Sept 9, 2010, we can change the UOM.   I have looked at the Transaction inquiry for the item.  No transactions appear.  I have used Purchase Order Summary searching by item. There are no PO's for the item.

Normally, I would expect that I  could change the UOM as there is no transaction history for the item. In the TEST instance, this is true.  In the PROD instance, I cannot change the UOM.