Last year in Wyoming I felt poorly for the first couple days...I don't remember our elevation, but we were in the Bighorn Mountains. We were running a cold camp(mornings) just so we could speed things up and get moving. In doing so, I was not drinking coffee in the morning. I can skip coffee here in Michigan, and it's no big deal.
Well, after dealing with feeling poorly and a massive headache, my partner (Mike Vines) said "drink some coffee dummy". I was cured with the first cup.
I'm not sure what it all means, but high altitude is no time to quit caffeine cold turkey.