My son has been travelling an average of 15,000 miles a year since he was 2 months old. When my son was in diapers and it was my turn to change him, I always always always took him to the lavatory to make the change.
I am 6 feet tall and managed just fine. The way I did it was to squat in fron of the toilet so my thighs were level with the toilet seat. This gave me ample "surface" to lay him down and do the dirty deed.
I would have never been so inconsiderate as to change him in a seat; especially with another passenger sitting next to me!