Air density changes with both elevation and temperature. What is of concern is how many oxygen molecules are in a gulp or volume of air. Higher elevation reduces density, and spreads the molecules father apart causing less of them in a given volume of air.
Higher temperatures also spread the molecules apart, yielding less of them in a given volume of air.
Humidity is yet another factor, as introducing/suspending water molecules in the air, displaces some of the oxygen molecules in a given volume of air.
If we knew what air/fuel mixture ratios you are currently operating, we could make a guess at what elevation, temperature, and humidity might make to your engine operation.
The worst case would be moving to high elevation, high temperature and high humidity. This would move your uncompensated fuel mix to the richer end of the scale, due to less oxygen being available to pair off with the hydrocarbons in the fuel. Conversely, low elevation, low temperature and low humidity, would move the uncompensated mixture to the leaner end of the scale.
Generally speaking, if your engine is running rich at sea level, it will be worse ascending to a higher level. Whether you will start losing extra power or sooting spark plugs will depend on current mixture settings, which no one on this forum can know without data collection.
A fourth factor is the barometric pressure, which also changes air density, as different weather patterns shift about the earth. However, predicting weather is likely beyond the scope of this forum topic.
Cheers,
