My lady (she doesnt have CF) and myself (I have CF, but very healthy), went to the zugspitz(sp) area in Germany. As soon as her and I exited the tunnel, I automatically felt the sensation of drowning. She did also. We both talked to each other and tried to breath, and then after a good few minutes, we were OK. But there was a time there where I almost paniced. It was because we come from Florida, which is technically below sea level. We appeared there, and we dramatically had things change from what we were used to, to 500 times higher elevation, and our bodies freaked out. If I wouldnt have personally "caught my breath" soon, I would have freaked out and went into convulsions...To me it was like choking on something solid that won't go away (like a jawbreaker or something), and i kept telling myself "Don't panic, you will be ok, this isn't you, just RELAX, things will work themselves out"...The whole time I felt like I was drowning. Thankfully I wasn't alone in my feeling, and my fiancee felt exactly the same way, and she has very healthy non smoking, non CF lungs. It's just a drastic change of altitude to those who come from sea level, to those who constantly hang out in that atmosphere and snowboard all the time.
We were fine after the panic sensation left both of us. We sat in the cafe and had a great meal, and watched all the snow boardes come in from outside (about -10 degrees farenheit)...But yes, to answer your general question, coming from a place thats extremely low altitude, to a place thats extremely hi altitude, its a massive shock to your lungs. If your lungs are already extremely comprised, meaning you already have massive lung damage and your capacity is very low, I'd highly not recommend that you go visit places where the environment is high altitude, and their air is very thin (one goes hand in hand).
That time was one of the very few times where I felt like I was feeling the way I did because of my lung disease, and the damage to my lungs. I'm sure my experiences were much more pronounvced that my non smoking fiancee, but knowing that she still experienced the same thing, I felt like It wasn't all me.