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Before you guys watch Korra you should know a few things, or maybe you've already started. Don't worry, this is spoiler free.
Avatar and legend of Korra are made by different people. Yes, the "creators" worked on both shows, but surprisingly few others did. If you look at the head crews of both shows, you'll notice they are very different.
Why this matters is because Korra is a very different show from Avatar, by virtue of being made by different people. A number of things from the original show get retconned, and some characters even get different personalities. This is because there are different people that are making the show, and this is their interpretation.
This issue isn't unique to Korra, many shows/movies/series change when they get a new director, producer, or if they're bought by a different company. (Like how StarWars was bought by Disney.) Some of these changes you might like; others you might not. Just don't feel you have to blindly accept whatever happens.