I agree, I liked it for that one episode, but I really hope it's not a permanent thing. First of all, seems like such a strange time to change to a completely different theme song and all the characters singing in that voice still freaks me out. It might've been cool too if they'd gotten all the voice actors to sing the song instead.
And yes, if they're going to continue to do stuff like this, I would love to see them try other genres. There are some upbeat alternative songs that could potentially work. Not so sure about metal, but I bet the show could pull it off at least once.