The core difference betwee CC and EC is how they are born... CC was born as "the game" at the time, EC came from a KR failure...
Also KR was born because when EA finished Warhammer Online they wanted to make the most of the engine they bought, so they went to the UO dev team and said: "Hey we have this pile of scraps from the other game, can you make something out of it?". So the dev team took hammer and nails and put together KR, but with their small team they had no chance of making something great, just something "MacGiver" style that sticks together with the dukt tape.
I remember when KR came out, the devs publish a patch every week, it was promising since if they kept that fixing rate that client would really had a chance.... but after a month or so they simply stopped (probably they run out of money?), and after that they simply started to chop off pieces of the client just to make it work (barely I'd say).
So you see? CC was born designed in day 1 by Richard Garriot and his team, trying to create the first MMO in history (so they wanted to have a chance), EC was created by a team who was a big fan of WoW and tried to turn UO into something similar with a bunch of scraps and a shallow knowledge of what this game was....
IMHO the best (and cheapest) solution is not to keep CC and neither EC, but to use the EC engine and start building from there something else that could please everyone... They could even just start by using CC sprites scaled up with an anti-aliasing filter applied (just so that you won't see the pixels)