It didn't actually change at all. The guy who put forward the idea of the 13th sign is actually an astronomer who decided suddenly that he was an authority on astrology. The system was never designed to be changed, and also isn't based on the position of the constellations- which is what the astronomer was assuming when he made his claim. The entire claim about the 13th sign is bunk, unless you're not using a western astrological system (basically everyone except Japan and India, who've had Ophicius or whatever as a part of their system practically forever)