It sounds to me like you’ve been “trapped in a barrel and tossed overboard,” so to speak.  Where is your champion?  He or she is supposed to prevent things like this from happening (i.e., sound project selection criteria).  Your project champion’s main purpose in life is to guard his or her flock of Black Belts.

Let me understand this:

1) You are a Black Belt.
2) Management defined a Six Sigma Project.
3) That project was focused on making some type of operational improvement.
4) The project was successfully executed according to plan.
5) The project benefit proved to be a reduction in headcount.
6) The benefit was not anticipated before-the-fact.
7) Headcount reduction is not an option.
8) Reassignment is not an option.

What’s wrong with this picture?  Assuming the situation is as stated (with no other complications), the answer is seemingly simple – keep the employee and fire the management.  Most likely, this action will lead to an even bigger improvement – sound business leadership, rational practices, and operational policy that encourages fiscal responsibility.