Don't judge the show by the movie. Just don't.
Well, if he liked the movie and now wants to see the series, I call it a win. If someone says he prefered the movie rather than the series, I call him insane. If he says he liked the series and not the movie, he's a fan. If he said he liked the first season but hated the others afterward, I call him a real fan.
IMO, I'm not that interested in that spin-off. If there's a new Avatar, that means Aang is dead. So are probably the rest of the Gaang, minus Toph who's probably an old and insane Earthbender. Appa is probably dead too... Zuko ? We may see his grandchildren, or face around. Sure, it could be interesting, but the feeling of the first won't be there. And they just CAN'T
replace Iroh. I'm sure we'll see a copy of him, but he'll be nothing like the original one.