well, according to the engineering website: http://www.engineeringtoolbox.com/ai...ure-d_462.html, atmospheric pressure at 8500 ft is about 10.7 psia (absolute pressure in pounds per square inch ). At sea level, the pressure is about 14.7 psia. The difference in pressure is 4 psia. The diameter of the neck of a standard wine bottle is about 3/4", giving an area of 0.44 square inches. The force on the cork is pressure times area, or about 1.6 lbs. I don't think that will give you any trouble, it takes me more than 1.6 lbs of force to remove a cork. Besides, shouldn't it get a little warmer at the lower altitude, and increase the pressure slightly?
tl;dr, it'll be fine.