They'll probably drop a couple of the episodes that don't really contribute to the main story, like "The Great Divide" and "The Fortuneteller", and maybe compress a few others down to their key scenes.
And as for the characters being introduced early, they're probably just extended cameos meant to tease the long-time fans of the franchise and get them excited about season 2. Though it wouldn't surprise me if they find ways to integrate them into the season 1 storyline. (For instance, maybe Azula leads the invasion of Kyoshi Island, instead of Zuko.)
Frankly, I'm fine with them streamlining the story a bit, so long as they keep the heart of it intact.