You want to change the document type or the vendor in service documents. The field is however gray and not ready for modifications.
Please check FAQ note 634395 question 15, listing all relevant fields. The information is valid for service PR and PO as well:
"The following fields cannot be changed subsequently due to technical reasons:
- Document type (BSART)
- Vendor (LIFNR or SUPERFIELD)
- Document date (BEDAT)
- Purchasing organization (EKORG)
- Company code (BUKRS)
- Language (SPRAS)
- Item category (EPSTP or PSTYP)
- Plant (WERKS)"
Related notes / content:
634395 FAQ: Purchase orders in the service