i asume you are not talking about keeping specifically this flat stripe in the middle out of whatever stylistic reason which would be easy to implement.
so i explain generally what is happening here. when you use the command FlatShade which works as a toggle, it will look like on the left if you switch if back on it will appear like on the right.
its actually just a display effect in which a texture gets calculated applying a visible softening, meaning the real geometry is actually what you see when you turn this softening off.
this shading gets lost when you convert something to polysurface because each polygon will serve as a seperate surface. if the shading is what you need because you are intending to render this you may have to reapply this as a phong shading but i dont know how to do this in rhino.
in software like c4d and probably blender or whatever other mesh focused software, you can add a phong tag which will reapply this, letting you set the angle tolerance. but again this is only visually, if you want to 3d print this then it does not make any difference even though it appears so.