I'm fairly sure this is actually a pretty common thing for Eldar in general (dark or otherwise). For certain gendered roles (such as Wyches or Aspect Warriors), an Eldar taking up one of those roles will transition to the gender of that role (so all Dire Avengers are male, all Banshees are female, etc).
TIL that there's a canon trans Drukhari Wych Succubus. Like actually stated that she used to be male, but after reaching this rank she transitioned into being a female Drukhari and seems to be totally cool with it.
Slay.