The title of the thread and your question are in conflict.
IIRC CR is just that, the ratio of the volume of the cylinder at BDC and the volume of the cylinder at TDC. So the CR would remain the same regardless of altutude as it is not a factor in the calculation.
Do the manufactures make an engine that has a different CR for high altitude use? I don't know, but I suspect not.
Do CO engine builders add to the CR when they build strokers more than their sea level counterparts? Don't know that either, but it is an interesting possibility.
If you're looking at a PSI number, then look to Fred's answer.
