🚀 go-pugleaf

Could someone please help me stretch/modify a GraphicComp in Unidraw using a
Command instead of a tool?  

I basically want to stretch a rectangle component so that its height is doubled. 
The user would select the rectangle, pull down a menu, and select a menu entry
called "double height".  This executes a command that I have defined. 

Ideally I just want to compute the height of the rectangle, add half that height
to the top vertices and subtract half that height from the bottom vertices.  I
have gotten to the coordinates and computed the height etc, but I do not know how
to change the coordinates.  There doesn't seem to be a member function besides
the constructor that sets the coordinates.

So far, I have been able to move the rectangle, rotate it, change it's
colors, delete it, etc. but I do not know how to just simply alter the 
coordinates.  I tried to use the "ReplaceCmd" from Unidraw and it segmentfaults.

Thanks in advance for all replies!

-Zak.