Use the familiar pet palette commands to edit the Zone polygon.
See Reshaping Polygons and Chained Elements, and Offset All Edges.
Offset Edge - Fixed Area
Use this pet palette command to offset the clicked edge while keeping the Zone area constant. This is useful if you are doing space planning based on defined areas.
In this example, you offset the right side of the Zone - and the shape is automatically constrained so that the area stays constant (as seen in the tracker):
Zone Interactions on Section-Elevation
Use pet palette commands from selected Zone nodes, even if the Zone is cropped: Drag, Rotate, Mirror, Stretch Height.
If you use Stretch Height to modify the bottom of the Zone, its sub-floor thickness value remains unchanged.
Stretch Height of a Zone in Section