Today is my birthday so I’m keeping things short.
Koopas get a flying upgrade just like the Goombas did.
Sometimes they just hop and flutter about and sometimes they fly in left-right or up-down patterns. They also take an extra hit to defeat. (or more if you can’t deal with the shell)
Oh and you get giant clumsy Paratroopas too.
(And now: birthday time!)
[EDIT: I was being factually inaccurate. Paratroopas were in SMB in both red and green. The only additions for SMB3 were the giant Koopas and the ability to pick up the shells. Birthday's make a man lazy.]