Hi Pooja,
I am afraid, I could not find anything to change the item type. Actually, item type determines item category (Standard, B or D) when the PO is replicated in ECC and therefore SAP does not allow to flip it on approved shopping carts as it may cause a loophole in process.
The SC need to be recreated with the changed item type as per business requirement.
Regards,
Ravi Pachauri