Also, this may not be clear to new users, but a rectangle, ellipse, and QuickShape are live objects.

In the case of the rectangle you can apply curved corners and go back next week or next year and edit the curvature. (I'm not real sure about more than a year because Xara has been changing things a lot of late).

However, once you convert any of these objects to Editable Shapes, then the live effects are removed and you can no longer automatically edit the curvature.