I have been working with doug fir and sub-alpine fir and can pass along some observations about these western U.S. species:
First off, you might notice some of the needles of your tree turning yellow then orange and brown and dropping off. These are the old needles and this is normal for a fir to do this. Don't be alarmed if your tree does this as it's part of its natural life cycle. Fir is a tough species and as you mentioned finding yours at elevation, I would recommend wintering it outside on the ground with the protection you give for it being mainly from the wind. You want it to be covered with snow as this is what it's accustomed to and is an excellent insulator against the elements. Place it in a calm spot near your house or garage where you can keep an eye on it through the winter.
With respect to re-potting, spring is always the safest time when working with species we aren't familiar with. Watch the buds and when they begin to swell and turn slightly green is the best time to re-pot. Treat it like a pine and carefully remove 1/2 half of the field soil the first re-potting and the other half the following year. This way you are not bare-rooting the entire root system and the mycorrhizae will have a place to start colonizing from. I hope this useful to you even though it is not for balsam fir specifically
