I went back to the comments about the previous episode before watching this one, and I found a lot of parallels between what we were thinking / speculating back then and what the characters went through in the hours after they found Sophia. There was a lot of sadness for the girl, confusion as to how that happened, and a fair about of shock at the revelation (for those who hadn't been speculating as such for the two episodes prior).
Thing is, we've had a couple months to digest the events and wonder what will happen next. When we pick up here, it's JUST HAPPENED and at most only a few hours have past since Sophia. Most of the characters are in shock from the revelation and/or realization of the futility they'd been going through for the past days (or weeks/months in the case of Herschel and his brood). EVERYONE was handling it differently, and this episode went through that period of adjustment and introspection very well.
As for Rick, maybe for him it DID signify a change in character. He was desperate to appease Herschel and get him to let them stay so they could find Sophia; now he's on edge, and he very quickly kills the first two humans they've met since they got to the farm (even if they were telegraphing their intentions to begin with). Perhaps as Glenn suggested earlier, finding out about Sophia has lifted the big thundercloud of guilt over his head and he's moving onto his next thing, trying to keep Herschel around not for his ego but for Lori and the unborn kid.
Carl's short time here moves his character in the same direction as it goes in the comics. He's had precious little innocence since things began and here he shows how cold he's becoming to the world that was. If they ever get to their next couple destinations as outlined in the comics (and who knows - they may never, given how much this series has diverged from its source material) then Carl will have a lot to NOT worry about when they get there.