gotta a question for the experinced reloaders, Let's say I dial in a really nice accurate load with good velocity here in the desert but then I hunt up in the mountains on much colder weather. I realize I will get an increase in downrange punch and less drop due to the thinner air. BUT what about barrel harmonics, will the change in location affect the group size?

Check 0,1 and 0,2 grain up and down from the selected load if the differences are small the load probably will be fine in various conditions. If not select an other load.

My rifle range is at 6800 feet elevation above sea level, average temps when I shoot range from 10 degrees to 90 degrees, mostly dry, sometimes windy, and sometimes rain, fog or snow.
This is where I develop all my loads and shoot all my groups. I've often wondered what changes would be apparent with my loads when I hunt higher and colder ( I shot a Mountain Goat at about 13,800 feet above sea level ) or lower and hotter (I've hunted Texas at 200' above see level and 95 degrees in temp) I also hunted in Alaska with temps in the 30's and 40's and elevation of 300' or so.
I have not shot groups or over a chrony in any other locations but I have never seen any pressure signs on primers or brass and never missed anything that I could attribute to point of impact change from temp or altitude, however this is mostly opinion and un-scientific speculation.
One way to give yourself an idea of the effects is to run a ballistics program, I use Sierra, and make changes to the elevation and temps in the program and compare drops and the like on paper just to see what minimal differences you may be dealing with.
As for a change in harmonics I'm having a hard time envisioning an environmental aspect that could change them. If the rifle is adequately bedded so that wood won't swell and if nothing has changed I can't see an effect. If you have water or snow on the barrel this would make a change no matter what temp or altitude you are at.
Good luck in your pursuit!