Personally I Love The Idea Of Character Growth Being Shown Externally - Tumblr Posts

Did the uppermoons appearance change when tanjiro brought them back as half demons?

Not initially, but as they’ve through their character arcs, their outfits change to signify and symbolize their growth. Think of shows like Steven Universe or Adventure Time. They do this with their characters as well. For now, I will keep their new designs a secret for now as to not spoil the surprise. Thank you for asking!