'Can Change Approval Hierarchy' for 'purchase requisition' document type does not seem to work ?

Hello,

In the document type screen for purchase requisition we checked 'Can Change Approval Hierarchy'

Now in iprocurement, before submitting a requisition, Oracle is not allowing us to change from default approval heirarchy to another heirarchy.

This work for blanket purchase agreements,however it does not seem to work for purchase requisitions ?

regards,

Kate