Hold Material PO Vs Hold Service PO Functionality

Dear All,

For Held Material PO I am able to change the Doc. Date and vendor code, but for the service PO same is not applicable.

For service PO after pressing the Hold button both field are gray out.

Why system is not allowing changing vendor and doc. Date in service PO while in material PO same can be done.

Is there any configuration is missing at my end to enable this scenario.

Regards

Purvi