It is no good for anyBODY to tell you what to do... you should 'listen' only to your own spark plug color after doing 'plug chops'.
You can read how to do them in the "How to fix your Moped"
click 'resources' and then 'articles' above.
And YES... high altitude generally means to go leaner (from low altitude) because the air is less dense.
(but not if you are already jetted correctly for high alt)
And no matter what you do (short of a turbocharger)... no engine makes as much HP at high altitude as thay do at sea level.
This is why the US will have trouble attacking terrorists in the high altitude of the Afghani mountains... some of the choppers we have can't even fly that high.