Switching moons is a choice (like the other tournaments) and the winner can't change characters (like all other games). I'm unaware of any tournament rules in any game that allow the winner to switch characters.
It's weird that you're complaining that you can switch moons when you win, even though you can't change characters, then complaining that if you do win and want to change moons, you may be forced to learn a moon you don't like.
I'm just running with the community accepted rules.