How to convert a color scketch in a PDF document to a gray scale image using Acrobat Pro 9?

Suppose a PDF file containing a color sketch has been created using LaTeX. The code is given below for completeness only.

\documentclass{minimal}
\usepackage{pstricks}
\begin{document}
\begin{pspicture}(6,6)
\psline[linecolor=red](2,2)(3,3)(4,2)(2,2)
\pscurve[linecolor=green,linewidth=2pt,%
showpoints=true](2,2)(2.5,5)(3,4)(3.5,5)(4,2)
\pscircle[linecolor=blue,linestyle=dashed](3,3){1}
\end{pspicture}
\end{document}


The image thus created and saved as a PDF is given below.

Suppose we want to convert this image to a gray scale image using Acrobat Pro 9. The question is how do you do that?

The following are some data of the file and an output of my failed attempt.

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You should mark your answer as the solution. –  Lèse majesté Dec 18 '11 at 5:41

By trial and error, I found the following solution.

(1) Open the file in Acrobat Pro.

(2) Select Export > Postcript > Postcript

(3) Select Settings

(4) Select "Generic Gray Profile" for Color Profile.

(5) Save the file.

(6) Now open the .ps file (either in Acrobat distiller or in Preview) and save it as a pdf.

The following is the output.

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