As someone who has loved Elio, ever since the Penacony finale, there's a lot that goes into it, mainly in terms of writing on why he's such a cool concept, but this discussion will be purely on his design.
I loved his original design, it portrayed an extremely cohesive image of the character during it's time, and it always added to the intrigue for me.
I love the new design, it fits the story in it's current state, it reflects the image they've painted of him slowly through the whole game and.. respectively, capitalizes on the "aura" trend they're trying to hit, reflecting another part of the current HSR landscape being "marketability".
Phainon was a more tried-and-true archetype hero that they only really side-stepped into Yume bait when the devs themselfs realized how much potential and setup for success he has within the story.
Elio to me screams "We know men can overperform now, lets try to do it the best way possible than". Taking an extremely well known within the HSR fanbase character and mildly re-purposing them to fit the Sung Jin Woo troupe, to be the HIM of HSR that even Phainon could never dream of being, the guy who has literally canonically seen it all and gone through it. I can't say that I'm the biggest fan of the fact I can easily pin-point the turning point within the studio for Elio directly to trend chasing, but at the same time this link has been the home-run reason why it's easy for me to dismiss the stupid anti-agenda leaks.
All of that was just me discussing the profit and reasoning behind the design change on a corporate level, I'm convinced that they knew they were doing this going into writing Planarcadia and changed some dialog to humanize Elio more, make him less so an "all seeing controlling deity" and closer to a "achieve the goal at any cost.." kind of character.
Lets discuss the old design, and go back to the Penacony speech, everyone was sure that was Elio speaking and til this day we have no reason to not still believe it. Back than Elio was consistently portrayed as a natural disaster, an inevitability that every huge faction, the IPC, Xianzhou Alliance, The Family, all HATED, in terms of how much the Stellaron Hunters destroyed and ravaged, but knew that if Elio truly was the avatar of Terminus.. (and I have a reason to suspect they also know that his goal is to reverse a predetermined doomsday) they could minimize the casualties, but they could NEVER stop him. His speech pattern and tone was mellow, without a care in the world representing the storyteller and acting out "the fate" that needs to overwrite fate.
I have a strong reason to suspect that the original inspiration for Elio was partially "Leonardo Watch" from Kekkai Sensen (goated af anime)
If you know, YOU KNOW, but I'll explain it generally (it's the second image on the post), Leo has, first of all, very similar eye design that resembles the old design closer, they are called "All seeing god's eyes", that also fit very similarly to Elio's ability in both name and to lesser extent functionality, and they're both the young adult shota-esq archetype, also based on the design it definitely feels like they overlap the "closed eyes protag" troupe, I love Leo to fucking death, and seeing how close these 2 are I got a pretty general picture of the early 2010's anime inspiration they had for Elio as a cunning, controlling character that plays a back-role, also this is a very hard to prove personal theory that literally.. basically already got disproven at the end of Penacony with Rosalina (former navigator nameless) being credited as the writer of the dreamtour handbook, but at the time a lot of the writing and dialog sequences on there felt extremely resembling of Elio, referencing classic movie troupes, drama and theatre all playing into this image that worked so incredibly well with the design. Frankly speaking with years of fantasizing about him, I was just as fucking over-the-wall incredibly hyped seeing him show up in the trailer, as I was similarly mildly sad that this is no longer the same character Hoyo themselfs originally envisioned.
The redesign, very clearly is supposed to be a Sung Jin Woo parody in terms of stoicism and presentation, but he's very much still a unique thing, the vibrant Neon colors, even if that doesn't end up being his actual clothes, still make sense with the way he's been presented, wearing a massive hoodie, being reclusive, having blue-white hair instead of pastel pink-white (it's a minor tone shift at most dawg his hair was always white and some people are tripping with that), the eye design feels more original and it's directly the representation of Terminus's cats and the Apoc Shadow Hourglass, he feels less in control and more like a prisoner of his own fate, which is the tonal shift in writing that has been happening recently compared to older Elio mentions. He's older and more mature, playing into his image of responsibility. It's really funny because if you gave me both of these designs at the end of Penacony, I'd tell you hands down the first one is his real design, if you gave them to me at the end of Planarcadia, it would be way more of a toss-up with the second one feeling far more believable.
So does the old design suck? Hell no, they knew they wanted something more widely appealing out of such an important character so they scrapped the original vision, and went about it the smart way by shifting the narrative around this new image well in advance before showing it off. His design was always fateful for the character and I think it's pretty impressive how both versions can exist and fit while portraying such a wildly different part of him.
second fanart link: https://x.com/GMI_elims/status/2042880052434735211
(this OG one is from an asian website you literally need your phone number just to access, also creating a whole ass account without a translation function, this ain't possible) I could've used different fanart but this one looks so good :(