The upbeat version makes its first appearance right after the Xindi attack, iirc (or maybe it's after they learn the aftermath of it, because the attack itself was a S2 cliffhanger), which cracks me up every time. Death, destruction, we're going to war! And then that acoustic guitar starts strumming a jaunty little melody. I lose it every time I do a re-watch.

Season 3 was where it got it's beard.