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When using "convertpstyles" to convert from ctb to stb I am asked to select a stb file. When I do I am given an error that says the color mapping table I have selected does not contain a color mapping table and cannot be used to convert the drawing.
See this link and look for Sinc's post. It has an answer to your question. http://forums.autodesk.com/t5/AutoCAD-Civil-3D/Converting-stb-plot-style-to-ctb-plot-style/td-p/2739...
Let us know if it does not work for you.