The reason for this behavior is the active initial user status in the item of the transaction. When the item category is changed, the system can only change (or delete) the status profile if there is no user status set for this item. If there is already a status set for an item (Say, E0001 'Open' is active), so a change of the user status profile is impossible. If you need to change the item type, then you have to make sure that there is no active user status in the item.
What is your idoc trying to do? update an order?
You need to check if the item categories are using crmord_i as their status profile. Mostly like not.
Spencer