I plan to fly in to Jiuzhaigou in Sep.

In JZG, for most people they won't feel sick or uncomforted. I had been there once, I saw many old people enjoy their sightseeing still, without any problem. So you don't have to worry about that.

We have been for 3 days at 3000m+ now, coming from Europe and having spent a couple of days in Beijing. We all had some minor signs of altitude sickness: headaches, dizziness, no appetite, out of breath. I was surprised nobody warned us about this. My family got over it quickly but I feel still dizzy and lightheaded. Nothing serious though.