Why user cannot update the default value in the Charge Account field after Checkout in the Punchout

Why user cannot update the default value in the Charge Account field after Checkout in the Punchout process.

In IProcurement, user went thru the normal steps for Punchout process. All is fine. But after Checkout, there is a default value in the Charge Account field. User wants to update this field but it's grayed out. He is not able to update this Charge Account field anymore. He used to be able to update. Another user( working in the same dept ) is able to update this field . Anyone has come across this issue before ? I think there is something for that user. Not sure what has changed.