WildStar, you'll be able to do so with the next major update. But it's going to cost you a pretty penny. Players have been a little upset about this for understandable reason, so Social Systems Lead Victoria Dollbaum took to the forums to explain why the prices for this feature -- as well as wallpaper options, atmosphere, and similar points -- were being set so high.
Dollbaum explains that rather than simple changes, these purchases unlock the feature forever, rather than incurring another cost each time you decide to change. While the current system is less expensive, it also requires you to buy changes to wallpaper, sky, and so forth every time you change; with the new system, once you've bought it once, you'll never have to do so again. It doesn't fully ameliorate the issue, but understanding the reason might make it a bit more palatable.