Can't change origin of a dimension in Drafting once it's placed

 Once placed on a drawing, a dimension origin can't be changed

  If the dimension origin can't be edited, right click on the dimension and Select Settings.

On the left expand: Line /Arrow

 Highlight "Arrowhead"

 On the right under Workflow, UNCHECK  Automatic Orientation, OK and the dimension origin can be moved.

 To set this for any new dimensions, select Menu-> Preferences-> Drafting.

 On the left expand Common -> Line Arrow -> Arrowhead.

 On the right under Workflow UNCHECK Automatic Orientation. OK

 Now any new dimension can have their origin edited.