I have seen masculine acting hens and my mom had one that would crow. but never seen one change gender in appearance. I've heard that it is not uncommon for a hen to take on a leading roll like a rooster if there is a hen only flock but this is somthing else. Strange
We have a large white rooster that for a year we thought was a hen. He is a year old and we just heard him crow for the first time, and he is just now getting spurs. Kinda crazy. Not sure on the breed to be honest. Might be a white rock.
In some birds, a female will have a malfunction in the ovary, the hormones go out of wack and it will take on a male role. Not exactly a sex change, but a change in behaviors. It's been a few years since I read the study while writing a paper on fowl and thier applications in lab work, so I don't remember if the hormones cause the change in ovary then in behaviour or if the ovary dies off first.