To Modify the Radius of an Arc Without Changing Its Endpoints

With AccuDraw active, select the Modify Element tool.
Select the arc.
From the Method option menu in the tool settings window, choose Radius Preserve Ends.
The drawing plane origin moves to the center of the arc, and the coordinate system aligns with the view axes.
(Optional) Rotate the drawing plane axes.
(Optional) Preview the modifications by keying in a radius value in the AccuDraw window.
Enter a data point to complete the modification.

Modifying the radius of an arc without changing its endpoints.