The Intellipap Auto automatically adjusts up to 9000 feet according to the manufacturer.
Even if the machine blows the "right" pressure, changes in altitude may have effects on your pressure needs.
1) At 11,000 feet, pressure drops by 30% or so. (that's 300 millibar or 300 cmH2O). It's enough different that the sensors or fan action may not work quite the same. I suspect the flow sensor readings may change, too. The pressure won't change by 300 cmH2O (duhh), but it may change a bit. CPAP machines are usually designed to self adjust, but they're usually not specified to go that high.
They'll probably still work, but may go out of spec.
2) The pressure difference is enough that there are some "mechanical" changes in the physics of breathing. Less O2 per liter of air, and some other changes. This might mean you need a different "real" gauge pressure. (Gauge pressure is the pressure in the mask relative to surrounding atmospheric pressure.)
3) There are also physiological/neurological changes in breathing and sleep apnea at altitude. These effects are not that well understood. There are complex control mechanisms in your breathing system. Your CPAP pressure needs may change. In particular, some people develop central apnea at increased altitude. Other changes happen as well.
As I said, the effect of altitude on apnea/CPAP is not really that well understood.
Personally, I wouldn't be too worried, but there's a possibility of some problems.
Average barometer readings vary around 2.5 millibar (mb) or higher during a day. 1 millibar is approximately 1 cmH2O, so that's 2.5 cmH2O.
I just looked up the National Weather Service records for New York, http://w1.weather.gov/data/obhistory/KLGA.html
, and the pressure has varied from 1006 to 1021 in the past few days.
If the CPAP machines pumped a fixed absolute pressure, the effective pressure on your lungs would vary a few cmH2O during the average day, and would have varied by 15 cmH2O in the past few days in New York.
CPAP machines measure or produce "relative pressure," which is what your lungs "see" in terms of how it affects your breathing. Without that, they'd be useless as the weather changes.
Most pressure sensors are inherently differential or "gauge" pressure sensors. There is some sort of element that flexes based on the differential pressure between two chambers. In order to make them read absolute pressure, you just seal off one side to a sealed air chamber. It's actually a little harder to make an absolute pressure sensor than a "gauge" sensor. It may be that the manufacturers sell more absolute gauges, but it's very easy to make a "gauge" sensor that measures relative to ambient pressure. You just don't seal off the "back" side of the sensor.