There is quite possibly a chart in your poh giving you the information that you need.
Yes power at a given rpm will change depending on altitude. Imagine taking it to an extreme. If you managed to get so high that there was almost no air left it would take almost no power to turn the prop at 2500 rpm as there would be no resistance.
Yet on the ground it takes quite a lot of power to achieve the same rpm.
Last edited by dublinpilot; 13th Sep 2012 at 17:52.