Item Master UOM Change in Oracle Apps R12

We are trying to enter a secondary unit of measure for few items and system is not allowing that. Secondary uom field is greyed out. When trying to change the Tracking uom (so that we can enter secondary uom), it is giving an error in item master for master org saying you cannot update this field when there is onhand quantity or transactions exist.
The reason for entering a secondary uom being, we have a customer who send PO to us in Case uom. When sales order is entered, we want to enter the order lines for those in Cases. Right now the only uom for those items are Each (EA). So when we return acknowledgement, ASN and invoice customer wants the uom to be in cases rather than in EA.