Glory to Ka!
Why did Green Dragons change to acid in 3e ... ?
Because maybe someone at WOTC back in the early 2000s decided that chlorine gas would mix with the dragon’s saliva and become hydrochloric acid? It’s just a thought.
How about the new (old) ones in Fizban?
I mean, some of them are pretty cool to me. My personal favorites are the Dragonbone Golem, Crystal Dragon, Topaz Dragon, and Sapphire Dragon.