User blog comment:Lauren Darkmore/The RWBY AMA on Monty's twitter/@comment-10390252-20131111000031/@comment-918376-20131111184100

Maybe there's no legal age limit for changing one's name (for example, at least in the U.S., one has to be 18 or older to legally change one's name) -- one of them (probably Yang) wanted to change her name and so *bam*, different name.