I've heard that the UI was drastically changed but I always wondered to what extent it changed?
The menus have been made friendlier and easier to access what you need without jumping through hoops. In addition, you can fly around with your mouse (meaning, ship turns to aim at mouse pointer) in addition to being able to 'free' the mouse to click on stuff.
So is X3-TC compatible with X-Tended or did they make a new mod for it already?
To be honest, I don't know. I haven't played it in quite a while, just found the screenshot of the Terran Odin.
I didn't read anything about it, but did they modify the gameplay so that it's easier to destroy other ships?
Well, having mouse control over your ship makes fighting quite a bit funner since you don't need to pilot with one hand and aim with the other. There's also a boarding mechanic in TC - you recruit a bunch of guys and train them in several skills to make boarding ships easier/faster/better. First you bring the ship's shields down, and have to keep them down so when you release your guys in space suits, they don't fry on the shields. Then they have to cut through the hull (skill-based), fight the enemy crew/security while making their way to the bridge (skill-based), and finally override the ship's controls (skill-based). At pretty much any point you can fail, your boarding party can die, etc. It's quite fun, and the TC universe is littered with unique ships that you can't buy, only capture. For example, there's a really awesome M6 that can actually hold 2 fighters, sort of like a mini-carrier.
The other big change is that while in Reunion you had to dock with a station and browse through news postings for jobs, here as you fly by stations they will have an icon to indicate if they have a mission and what kind of mission it is (kill, transport, build, etc). Then you just open comms and talk to the person offering it, no need to dock.