Enabling Lot and Expiration attribute to existing inventory items

Hi,

Since from implementation, we are using the item code without Lot enable and Expiration attributes. Now business is requesting to enable these attribute to track the inventory item correctly. These attribute we need to enable for few of the items. System is not allowing to enable these attribute on item definition because of on hand quantity available in the system. Upon further review it was found that all these items under reservation against very old transactions (internal sales Orders) in closed financial periods. We are not in favor to define new items because of on OPM formula and receipt.