Seemed pretty clear to me from the beginning that you can't:
http://forums.d2jsp.org/topic.php?t=38127392&f=272Quote (njaguar)
Here are some of the changes/addons in this version:
- Female characters have been added! You may choose female when creating a new character. On any existing character pre 1.18, you may type (once) /sex 2 to change your character into a female. This command is permanent!
He said relatively recently that he may change things to allow people to change their sex back, but at the moment it's impossible
