It's pretty easy to make a first stab at how conditions might be, at a different altitude. Rather than go through all the psychrometry and the maths, have a look here:

http://www.engineeringtoolbox.com/ai...ume-d_195.html
The BC or the drag coefficient whatever you want to use is in inverse linear proportion to the density. So if the density at the elevation you are going to shoot at is say 1.05 kg/m3 compared to your set up site of 1.2 kg/m3 then expect the BC to be higher, by that ratio. If the "home" BC is 0.025 then the "away" BC will be 0.025 x 1.2 / 1.05, which comes to about 0.029 to the same precision.

This simple start ignores temperature differences; if you want to include all the parameters then some maths will be needed. It also ignores humidity diffferences which are second order.

A full spec job will use the partial pressures of air and water vapour: go here

http://wahiduddin.net/calc/density_altitude.htm for a read-up.