Core assets as in the central aesthetics of the game. They didn't throw away their ship design and replace it with something new, they just improved the implemented textures, which really doesn't take that much time to do.
If you haven't tracked them down yet, find the GI scans and take a look at the armor designs. I don't recall Orcish armor being famous for accentuating cleavage, and I'm pretty sure House Indoril isn't fond of covering their soldiers in sharp-angled geometric shape armor.
Well, there's not much wiggle room for the ships. Players want to fly their favorite ship from the shows, and the devs always did their best to keep them canon. Overall, players are very happy with the ship design. So maybe that's not a good example.
About the armor - the Indoril armor seemed fine to me, albeit very low poly and resolution (like most of them, urgh). About the cleavage - well, I can tell it's a lot easier to convince developers to remove cleavage than to add it.

I also know of asian mmos that adjusted body shapes after beta-testing. I really think many of the assets we saw in the screenshots aren't set in stone yet.
The "core" is what the game is built around. Take WoW for example. You have tanks, dps (ranged and melee), and healers in a highly scripted PvE centric game that has point/click/auto-attack comabt. The classes that comprise these rolls (and the balance that governs the game) changes from time to time. That is your "mechanics change all the time." The core however, doesn't change. Thats the part where you have a tank, dps, and healer in a highly scripted PvE centric game that has point/click/auto-attack combat. The classes that comprise these roles may change from time to time (vanilla wow only had warrior tanks, as did burning crusade for the most part. However come WoTLK we had 4 good tanking classes) but the core is in stasis.
Well, that sounds like "the core is that which doesn't change", which we can't define unless there actually are changes. Even point-and-click mechanics can be changes.