Disclaimer: I haven't played the DLC, and probably won't for a while.
On the whole, this is really more of a clarification than anything. Before: Auri-El went to Ada-Mantia to ascend into the sky as a planet, outside of nirn but inside the mundus. Now: Auri-El went to Ada-Mantia to ascend into the sky somewhere in Aetherius.
And it's new because it prevents him from being a variant of Akatosh.
Does it? These are transcendent beings we're talking about. We already accept that Time-Mac-Timerson can have two personalities and fight himself, why can't he be in two places at once? The dude is responsible for
the entire existence of time for crying out loud. I'm sure he's got some leeway with space.
Auriel was already a god by the time Lorkhan was killed, despite him having been a mortal Aldmer (Altmer?) who ascended. Think Vehk.
Dudes and dudettes, Auri-El, Alkosh, either Tall Papa or Sep, and Bahu are the only four timey-dudes who are explicitly said in myth to pre-date the mundus. Auri-El has the most consistent support for this. Akatosh is only mentioned at the start of this Kalpa (and is clearly an invention from later), while Alduin only exists inside the Kalpic Cycle, and Tosh Raka is a later historical figure. Yes, Auri-El walked the mundus and did all that Jazz, but that doesn't mean that he or his identity came into being as a mortal.
Waitwaitwat, doesn't this mean that Auriel isn't an Aedra at all? Is he Magne Ge or Daedric?
Depends on your definition. By the standard one used on this forum, he's Magna-Ge. By the original Altmeri meaning, he (and Magnus himself) is Aedra.
Remember that the gods around that time actually were two. See Shezzar's song. So Akatosh and Auri-El were seperate and the same. One decided to dracochrysalize, and ascend with his bro Magnus. The other stayed because he loved man. Actually, this probably happened to all of the gods. This explains why they are associated with Aetherius while being bound to Nirn. So the elven counterpart's realms in Aetherius probably house the souls of the loyal followers of their human counterparts in the Nine Divines.
That's a pretty rad theory.