It really changes the direction of the story. In ATLA the antagonist is a country, the Fire Nation, and their thought process, a Fire Nation manifest destiny (you might say). In the ATLK the antagonist is genetic group against another genetic group, nonbenders and benders.
This really reminds me of WWI and WWII. ATLA is very much like WWI because the antagonist was a country trying to annex new land. On the other side, ATLK is very much like WWII because it was a genetic group, the pure Aryans, against another genetic group, parasitic groups like the Jews (Later the Allies joined in). I don’t know if they meant to do this on purpose, but there are many striking similarities between these.
I really can’t wait for the series!