The term switch was something adopted by later more politically correct renditions of the bdsm comunity to be a gender neutral term for someone who is into both dominating and submitting.

It came into use via the the leatherman, who in turn got it from the male homo-sexual community who in turn adopted it from american baseball.

"A switch hitter" in baseball became initially in their circles a code for a man who liked to be the penatrator or Top as well as "switching" to be the penatratee or Bottom during sex.