Be sure to gradually increase your dog's exposure to higher altitudes by starting lower and taking short hikes at progressively higher elevations over the course of several weeks. If your dog is adjusting well, increase the distance in the same manner.
Your dog will get acclimated in a few days to high altitude. However, if you find that he has not acclimated and continues to display signs of altitude sickness, then take him to a lower elevation as soon as possible. If symptoms don't abate, then schedule a vet visit.
Even if your dog is healthy and has successfully completed high altitude treks before, there is no guarantee that they will not experience altitude sickness subsequently. Observe your dog for signs of illness whenever they are exposed to altitudes over 8,000 feet.
Your dog will get acclimated in a few days to high altitude. However, if you find that he has not acclimated and continues to display signs of altitude sickness, then take him to a lower elevation as soon as possible. If symptoms don't abate, then schedule a vet visit.
Dogs can survive a 12-hour flight, but it's important to take steps to ensure their safety and comfort during the journey. Before the flight, it's important to prepare your dog properly by providing them with plenty of food, water, and opportunities to exercise.
Dogs are routinely flown on small unpressurized planes without incident. There's even a volunteer organization that relocates shelter pets using GA aircraft and they do not have many problems. Just get a pair of dog ear muffs, stay below 10,000, and don't fly like an idiot, and your dog will be fine.