Just finished the first episode, it's different, I feel like I messed it up for myself for rewatching the og one only a few weeks ago and having that fresh in my head, how ever I am a fan.
I think it's important to go in with the expectation that this is a different story that adapts key elements from the og series. But it's not the same, it's different.
I'd say don't watch it so close together, theres a lot they are trying to cram in and it kinda misses the mark, not by much but it's still a miss
Legend of Korra is great if your looking for the next one to watch, it's a bit darker, but keeps the same feels, touches a few things form ATLA but very much it's own show highly recommend