I can't believe I am coming back to this. But I just want to say I agree with Lurking.
Although in fact I don't really care what other people do. I have had people ask me if I mind if they change a nappy in my vicinity on a train, and I have, of course, said go ahead.
I'm not offended by babies bottoms. I am offended by lack of consideration for others.
Imo, (if you absolutely cannot wait, or get off the bus) then asking the other people on the bus if they mind if you change the nappy, then retreating as far away as space on the bus allows, does show consideration for others. Saying "my child needs changing so I'll do it and I don't give a stuff what anyone thinks, if they don't like it that's their problem" does not.
As far as it concerns me, and my children, I would far rather get off the bus and try to find suitable facilities than change on the bus. It is more inconvenient for me. It is more considerate of the other people on the bus.
I don't think having a child is an excuse to ride roughshod over the feelings of everyone else. I am the child's parent, they are my responsibility, therefore I feel that if someone is going to be inconvenienced, it should be me.
And by the way, I wouldn't have any objection to changing a nappy on a park bench. The thing about a bus or train is it is an enclosed space where people can't just walk away if they don't like what you are doing, as they can in a park or wherever. So the decent thing is to walk away yourself, by getting of the bus, if possible.