A submissive becomes a switch the same way everyone else does, i.e. by realizing he/she enjoys being in control of another and being controlled by another. If a "sub" truly enjoys dominating other subs, he/she is a switch. If a sub is only dominating another at the instruction of a dom and not enjoying it at all, except in the context of pleasing the dom, then the sub is just topping from the bottom.
Think about it for a minute. The flip of a switch (pun intended) is to be in the presence of someone more dominant or more submissive than himself/herself. Sometimes, this can occur within the relationship if both involved are switches. I think it's a bit ridiculous to think we need 100 extra words to describe the same damn thing just because people want to feel they are somehow different or don't want to feel different at all. Guess what? You're not *that* different!
In my opinion, an alpha sub is only a sub in a polyamorous relationship. To be number one, there must be a number two...or three. It's the same as saying "first wife" where people practice polygamy.
You can call yourself a sub who likes to play or a dom who is selectively submissive if it makes you happy, but you could save yourself some syllables and just say what you are.....SWITCH.