I guess I don't understand. If the air density is less, then you will have less resistance to the air, and have a little more RPM. So are you saying that to maintain the same speed, you need less pitch? This will convert to more thrust at the lower density.
ORIGINAL: FHHuber
Actually... High altitude... you just want a little less pitch. Lower pirch gives better low speed POWER. the engine is mre efficiently using the propeller at high rpm with low airspeed. This gets you off the ground better.