I think it is a mistake in almost every case and for almost every fighter. Anybody that has done this long enough- hitting the bags, working out in gyms, etc..- has at some time or another become bored and taught themselves to box from the other side; and while you can do it there is never any illusion as to thinking you are proficient enough to do it for real. ( there are exceptions like Hagler or Chiquita Gonzalez, two that come to mind and even at that level they were better leftie than orthodox) The majority of times you see a fightwer switch mid-fight it is he that is either already confused and out of ideas, or about to be. The other evening I saw a guy box a smart and effective round 1, then come out southpaw for 2 and proceed to lose the fight. I think a boxer, and especially a young boxer, would be better served mastering one stance and all implied in that instead of trying to be confused. If anybody still remembered 'side-stepping' much less how to teach it this whole concept of "switching stances" would be pointless.