I put together a project using the Cinema HD working space and the client needs it in XYZ Gamma 2.6. When I change the color settings, all my colors turn into a day-glo super saturated mess. Is there an "easy" way to get my colors back to normal while staying in this new color space?
I'm not sure about going between your mentioned color spaces, but for going between 601 and RGB color space, there's a "Levels" presets included with AE. I think it crushes/expands it to 16, 235 (0, 255).
So... Perhaps there's a levels adjustment you need to figure out for your color space. Or maybe use "Exposure".
Does it say anything in the color management manual/pdf?
You could create adjustment layers with the Color Profile Converter effect and a Levels effect... All you should need to do is to set the color profile to the Cinema HD as input and your high-fidelity space as target, then use the levels to fine-tune.