Another Sherlock X Elementary crossover.
What is interesting with Elementary is Sherlock on that show was just sobering up. He was at his lowest point and it showed in his fashion. Hence the ironic, non-ironed shirts that barely made it passed the sniff test. ("Is this clean enough to wear?" *sniff* "Nope.") But now that Joan has come into his life and he has found a kindred mind in her, Sherlock has been starting to dress up. Almost to the point he has gone the opposite direction. (Buttoning his shirts all the way to the top, making him looked too stuffed shirt.)
Here it was Joan's idea to switch the Sherlock's fashions around. Just to see how it would look. That and she likes to mess with them. Of course, John is not going to object. Sometimes it's fun to mess with someone who that tightly wound.