Hi,
No, We cannot change the document type of DIR once it has been created. Since document type is the KEY field for any DIR and changing the document type which means it needs to change all other factor like status network , document type settings etc which is the reason system will not allow you to change the document type once it has been created.
I have a doubt even this cannot be achieved through customization/enhancement.
Please let me know if you could able to achieve though enhancement also let us know the business reason for this requirement.
Rgds,
Nayeem.