Big Frank wrote a short tutorial in the Xara Xtreme forum on how to resize boxes with inverted round corners without changing the curves: http://www.talkgraphics.com/showpost...02&postcount=4

This same technique can also be applied in XWD when resizing boxes with regular round corners, but before using the Shape Editor Tool to resize your box (Step 3) convert it by selecting Arrange/Combine Shapes/Add Shapes.