Isolate BReps based on height

Hi all. I have a collection of BReps that were made using a 3D Voronoi, and I would like to isolate them based on if they are "top" BReps or "bottom" BReps (you can tell based on which face of the geometry is flat). I want to do this basically to take all the "top" ones and vary their height while also being able to ground the "bottom" ones at the same Z value.

My initial guess was to DeBrep them and use the Z values from the vertices in order to isolate the BReps, but I don't know how to use this data to cull a list. Also using the area-based center point could be a useful way of isolating them, but again not sure how to cull the list using this data. Rhino file is attached.

Thanks in advance!