Well, think of how much Grissom changed from season 1 in the original CSI, to what he is now. Completely different, IMO. He's more closed in now, stuffier, less overtly emotional, etc. In S1 he got angry and showed it, he joked around, he flirted with women any chance he got, he mentioned to cath that he goes out to eat once in awhile (for calamari...sometimes with a beer, LOL). For some reason the writers decided to make many changes in Grissom, I think WP wanted him more "mysterious", from what I've read. So changing a character in CSI isn't unprecedented...
HOwever, at least with Mac, it wouldn't be "changing" as much, to me, as it would be "evolving". He finally laid to rest his grieving and demons re: Clair's death at the end of S1, so it makes sense he would lighten up a bit. Now he's faced down/laid to rest some of his demons from the painful experiences in the marines at the end of S2, so I would expect that would create some (more subtle) changes in him, for the lighter. Almost losing Flack, losing Aiden, that might make him step back and take another look at his own life, realistically. (Just as long as they NEVER take Pissy!Mac away from me, I'm fine with him cracking a few jokes, flirting with a few women, and oh yes, appearing sans tie, with more T-shirts, please! Or shirtless!
) Seriously, whereas I saw Grissom's changes as rather bizarre and not making much sense, I see the much more subtle changes in Mac as entirely more realistic and almost expected. JMO...
OH, nearly forgot: I read over at GSC last night that the Season 2 dvds will be out in stores on Oct. 3! I personally love the cover art...I think all Mac Fans will also like it, LOL... let's just say it's a Whole Lotta Mac going on, on the cover (if it's indeed the final cover).