Flattening an extruded mesh surface

I have a complex mesh surface that I am extruding down by a fixed distance. The bottom surface therefore is effectively a copy of the top( original) surface. I now want to flatten the bottom surface while still retaining the original mesh topology as the top surface. A Boolean subtraction simplifies the bottom surface so is not suitable. Can someone give me some pointers either in rhino or grasshopper how to achieve this. Thank you.

one of these might help to get the z-values of the vertices to the same Value / z-height.
you might need to set a
_cplane
first

then
_setPt Z=yes → w0,0,0
or
Gumball-Scale to 0
or
_projectToCPlane (copy = no)

be careful, if there have bin vertical Faces - you will get somehow invalid triangles (3 points on a line).

if this does not help - please provide more info, a file, at least a screenshot that illustrate why above did not workd.

kind regards -tom

Select the bottom mesh to flatten. Scale Z to 0. Or select all its vertices and use SetPt Z 0.