Note that converting from RGB to CMYK will usually degrade your colors because no exact mapping is possible. Anyway, this Stackoverflow post Show archive.org snapshot worked for me:

gs -dSAFER -dBATCH -dNOPAUSE -dNOCACHE -sDEVICE=pdfwrite \
-sColorConversionStrategy=CMYK -dProcessColorModel=/DeviceCMYK \
-sOutputFile=output.pdf input.pdf

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