hi Vasiliy,

Looking at the Apple Thunderbolt Display Teardown, step 2: The Thunderbolt Display also comes with a built-in Thunderbolt cable attached to a Universal MagSafe cable.. At the end of the tear down the cable is still in place.

Basically we can conclude that it is not user removable from the outside.

However Apple Thunderbolt Display Teardown, step 10: Interestingly enough, the Thunderbolt cable that routes into the display also plugs into a standard Thunderbolt socket on the logic board. Apple could have just soldered the cable wires to the board, but instead chose to implement a cover that prevents the cable from being detached from the logic board's Thunderbolt socket.

It looks like apple made it relatively easy to replace because there is a Thunderbolt socket inside the device. So one Can replace the cable if they open the display up with either the correct cable designed for the display or if you need a longer cable you can also swap out to a standard Thunderbolt cable, but you loose the MagSafe cable to power your laptop.

I hope this helps!