Are you saying that you will take your character that beat classic, with all their endgame armor and gear and try play a new game with expert? Because that would just trivialize 90% of the content. Personally I recommend beating Expert Mode with a new character before touching calamity, as calamity is very large and it can be quite confusing going through it without good practice in base terrraria.
Yes, it would definitely trivialize the content, but unlock Expert items for my current character. I would play Expert from the beginning on a different playthrough in the future, but I'm not looking to entirely restart at the moment, since there's still a lot of building and etc that I still want to do with this character. I was wondering if it would ruin my future Expert experience by doing this trivial run now, in which case I'd not do it. Is it an entirely different Terraria experience, or is it just that the enemies are harder?