I could not find a way to do this anywhere, so I assumed it is not in the game. I think that being able to change monster and player appearances based on their level would be a cool addition to visually differentiate lower level characters from the higher-level ones.
This might be tricky to support; we’re able to draw so many characters at once because there’s only a limited number of appearances for them. If they suddenly look different, they’d likely be a lot more expensive to draw!
But this is something that’s on my list of things to think about, to see what we might be able to do to give a stronger feeling of progression. I don’t yet have a really good solution for it, but it’s definitely something I’m still thinking about!