In past games as far as I remember you can't downgrade so, I would guess it would be the same for New horizons but you know they could add in but I doubt it.
Sadly I don't think so! It would be nice to pay like 30k bells and decide to downgrade the size. You can create a new character if you like decorating small houses but still wanna expand your main house
I wish you could choose your room size... but the first upgrade only allows you to make the first room bigger, not add another room. I want the extra rooms, but wish the first room didn't have to be so huge.