should also say, doubt it "costed him the win" he should have lost like that at some point in the fight (but that was true with Fury too) but switching southpaw tells the other guy that he wants to eat a straight right. I've seen guys switch a lot of times over the years, usually with no clear reason, maybe they're bored, maybe they just think it looks cool, whatever. It usually doesn't help, sometimes it seems to work for a moment and usually, the results aren't this disasterous but a guy with limited experience doing something that top guys can't do? Not a good idea.
Look at the greatest fighters to ever fight, most of them, I can't think of a single instance where Leonard, Ali, Robinson Louis switched stances. Some others like Duran was said to do the "fitzimmon's shift" and tyson apparently imitated it but those are still the exceptions. But gosh, don't it look cool? Like I told my buddy's kid who was hanging around some local euros who probably saw their hero doing it, I just told him it's not a good idea.