Steps ...
2 -- Create Tiddler called "Misery" With the "text" THIS IS CONTENT... Save.
3 - Filter to and select it ...
4 -- Navigate to Fields > Add fields. Field name:"type", field value:"text/plain". Click "Add New Fields ...
RESULT: The Content Type is correctly changed. BUT the "text" field is deleted.
UPDATE: Birthe pointed out it can work without destroying the "text" field so long as you do NOT fill in the Content Type.
I tested that and it is a WORK-AROUND to (a) Add Field "type" (but not content type), then (b) Set field value of "type" to "text/plain".
Best wishes
TT