Not able to select document style from Buyer Work Center

Hello,

We have set up service procurement to use the complex PO.

We are able to create complex PO from Buyer Work Center manually.

Now we are facing problem creating a complex from a approved requisition which have a CPA reference.

We are unable to change the document style to Complex PO, as the field is disabled with standard purchase order style.

Please let us know how can we able to change the document style to complex po while trying to create a PO from Buyer Work Center manually referencing a purchase requisition which has a source document reference.

Regards,