What's the same?
Same stuff
Themes still remain the same. The Underdark is still a thing. The pantheon and mythos is the same. Tyr exists as does Gond and others. And your favorite races and lovable items and spells.
These things remain mostly because they form the backbone of the world. Think of D&D as the framework and then adventuring out to Ravenloft. You're still using the same set of rules, but the location has changed.
Also, there's that guy over there on the right. He's a new addition for my world.
What's different?
Different stuff
City names, rulers, land shapes, locations, etc. These are changed because I want the world to be able to be modified by me, of course, but by the players as well.
This does happen, and can happen again. Will you play a part of this? Will you make a difference?