That was my initial reaction as well, but it was more a drastic change in speech style than personality. Tobirama and Hashirama got in a combined three sentences
before Hiruzen informed them that had been brought into a bad situation and should prepare to get fucked up. After that, they realized some jackass had used Edo Tensei to force them to fight their former student and bring danger to the village. It put a bit of a damper on things. This time, everyone either of them recognizes is dead except Orochimaru and they don't see the village or a potential opponent. The circumstances permitted them to behave differently.