You can convert polygons to circles by:
Setting the polygon's Number Of Sides parameter to 0 in the polygon's Edit Properties dialog box.
In the Browser, right-click the polygon node and choose Convert To Circle from the context menu.
Likewise, circles and ellipses are converted to polygons by right-clicking their nodes and choosing Convert To Polygon from the context menu. By default the resulting element is an octagon; you can change the Number Of Sides parameter in the polygon's Edit Properties dialog box.