Drag the control (blue square) to a different reference.
Right-click the witness line control, and click Move Witness Line. You can then move the witness line to a new reference.
NoteModifications to temporary dimension witness lines are saved on a per-session basis. For example, if a default temporary dimension measures the distance between the centers of two walls and the witness line is moved to measure to the faces of the walls, subsequent selection of the wall preserves the location of the edited witness line for that session.