The only flight restrictions regarding altitude are for areas withing 3-5 miles of an airport traffic area, and within an air traffic control zone. Those restrictions generally keep you under 400 feet above ground level. Within an airport traffic area, the control tower should be contacted to see if R/C flight is permissable in the area, and at what altitudes. I don't think the mountains will be covered in those areas unless a controlled airport is nearby.

Your planes will not fly as well at an 8,500 foot altitude due to what's known as density altitude. Without going into all the whys and wherefores, there are less oxygen molecules to support and lift the wings as there are at sea level. If the temperature is warm, there will be even less.

Your planes WILL fly at that altitude, but expect faster flight sppeds, and shorter glide distances. Control responsiveness will be somewhat less than you are used to, but you should be able to handle it.

Take the planes along and give it a try. It will be a little different, but still fun. We would all likely be interested in what the differences were in flight qualities.