Zenos is... [big fat spoily]

[Iscah]

To the common person in the street, if they know the world as anything other than "the world" or "the star" it would be only "Hydaelyn", so to suddenlly go around calling it 'Etheirys" to a random person in the street would just grant odd looks.

But we're not talking to "a random person in the street", we're talking to each other and we understand what the words mean. Everyone in the group of people our character regularly discusses such concepts with has shifted to calling it Etheirys. Why shouldn't we do the same?

Even for a newcomer to the game, if they ask why we're calling it an unfamiliar term, all you need to say is that later on in the game they introduce the term for easier distinction between the two. You don't need to explain where the term came from or why it's significant that we shift from one to the other.

Additionally the "Etheirys" jewellery set comes from the modern-day aetherial sea, not from the ancient world.

I am of the opinion that the new (or older, in this case) name is not necessary because there would be no confusion for the name Hydaelyn moving forward.

But we're not going to stop talking about Hydaelyn in lore discussions just because she's no longer an active character in the story.