In standard no such option is available, you've to go for an enhancement.
Try this:
You may restrict the document type field for changes in edit mode by using the following function,
Spro --> Material Management --> Purchasing ---> Purchase requisition --> Define screen layout at document level --> Select the appropriate field selection key (Eg: AKTV - Change) --> Select the enjoy fields and change the status of document type to display.
Please check these steps in a testing environment.