I personally would pay to have a Class change option in the Gem Store. I know I cant be the only person to have started and been on and off the game for years only to find that the class they have the most progression on is one that they are bored with... IE: I have a Sylvari Necro that I enjoyed like 5 years ago, but now I am looking at having to start from scratch just to try my Mesmer.. or to make a Ranger, and I hate the idea of losing all of my previous progress because I am bored with a class.
I get not allowing race change, that is tied to the Personal Story, but Class change wouldn't affect anything.