This one was easy. Bluehole likes leaving assets abandoned in place (aka lazy) so when they updated the Island of Dawn to be a level 65 hunting zone (with a story quest that ruined it), they left the old assets intact at:
<Your TERA Directory>\S1Game\CookedPC\Art_Data\Maps\ATW
They then added the ruined IoD assets into the ATW_Death folder (inside the above). It simply requires replacing the ruined IoD assets with the classic original ones (following the same naming except for the birds and floating castle assets).
DL:
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Speaking of classic, there is an actual way to get back the older character selection screen (Menma's TERA uses it). But changing this in the datacenter is going to be a problem since there's a lot of stuff that needs to be removed completely (and the tool doesn't work that way). This would be a piece of cake with an XML repacker.
Different regions may have handled this differently. When En Masse was running North America, their dressing room only had cash shop costumes. They used the fashion coupon UI to add in armor and weapon skins from the in-game drops which honestly worked out well (and before they eliminated the Tikat Shop UI/Federation Bills, there were some in there as well). I can now see why they tried to consolidate things because the backend is a mess (and unlike most of the other regions, EME didn't seem to develop a tool to aid in the dressing room issue since every update that Bluehole pushed out to production, requires it to be reloaded; I heard that EME used to hand edit which is why the last few years when their staffing got reduced, was an incomplete and chaotic mess).