I believe it was mostly just a change of boxes, which is a shame because I liked the first one better myself. The Tau and Space Wolf teams were both originally released as stand alone boxes, and then combined to make the new starter.
Kill Team is real fun. Easily one of my favorites of the specialist games. They did start pumping out more expansions then I was happy with, but I hear most of them are pretty good. One expansion adds the ability to have commanders or lower level HQ, and the other expansion adds elite choices like terminators. They make it a very different game then the low level squaddie stuff, but it also makes sense for some of the missions.
Kill Team was probably the most frequent thing I was playing before the pandemic.