It is good, but also predictable after it's been done once or twice if your opponent is aware of it.

What I like to do is turtle up, and after they've punched my guard a little I step back with my leading leg, thus changing stance, without the opponent noticing most likely because he is too busy focusing on my guard. The trick to using this Southpaw/Orthodox switch in a match, is distraction, and making it as smooth and unnotice-able as possible. That's the only way to keep catching the opponent off guard every time.

What you're doing is good, but after two or three times of doing it, a boxer who isn't groggy would probably notice. Try changing it as the opponent steps around you instead, when I do it, it makes them feel cornered, because I'm always directly in their face never giving them a chance to circle around.

Just try and be creative, there are tons of ways to do this without the opponent noticing, and if you learn how to switch at will and you have techniques for distracting them, you can keep doing it over and over in different ways and they will be confused the entire match.