Modify toolBox has a button called "Modify Element".
If I pick on a vertex and that Vertex= Rounded, then I can edit the Radius of the arc.
However, I would like to edit the Radius of the Arc without moving the location of the vertex so I think that the Tool Settings dialog should have a new parameter (checkbox) called "Lock Vertex Location". You could pick on a vertex but you were not allowed to drag it.
It seems that I must have enabled "toggle AccuDraw" to put the cursor on the center of the AccuDraw if I want to retain the same vertex.