You will be pleasantly surprised with how well they acclimate! I live in Northern Indiana and I've been to Durango twice with my horses!!!!!
I've found that it typically takes them 2 or 3 days tops to acclimate. Now granted, I make sure they are in shape before we go, but from experience, I will tell you give them plenty of oats and hay (don't bring anymore than you need for the trip there and back, you have to buy certified hay when you get there to take up to the mountains)
2 falls ago we were in the mountains riding a minimum of 10 hours per day for 7 consecutive days my paint lost at least 100 pounds!

You won't be sorry for taking them!