About an hour later, the smaller crab began to inspect/play with another shell (in the exact same way he has done in the past, right before he changes shells). We left him alone, and when we returned about 30 minutes later, he abandoned his new shell and was back in his old shell!
I'm not worried. We just found the behavior interesting. Guess he really likes the old one.
Yeah - my crabs will do this a lot. Especially when I add new shells or change up the tank.