http://www.dfn.com/agservices/jetfaq.html
Q#15:Why do elevation changes affect my jetting? A#15:In the same way air temperature changes the density of oxygen in a given air volume, so do elevation changes. As the elevation above sea level (0') increases, there is less oxygen per volume of air (lower density). Typically you'll need to rejet your mains one size for every 1500'-2000' elevation difference; jet smaller when going to a higher elevation and jet larger for going to a lower elevation (for example: 300 mains at 3000', 320 mains for 0' or sea level IF the temp is the same). Bear in mind that you'll need to compensate for BOTH temperature AND elevation changes, so going from a hot low elevation to a cold high elevation may net no jetting change. Much like temp changes, going from a low elevation to a higher one of the same temp will exhibit a bog at WOT because it's rich (dial in the mains-see Q#12); going from a high elevation to a lower one may or may not give you any symptoms of being lean before damage occurs. While there are other atmospheric factors that can affect jetting (like humidity, barometric pressure, etc.), temperature and elevation changes will have the most drastic affect.
good thing is once you get it jetted correctly you will have a good deal more available power at that elevation then you did at 5000 ft!