
Originally Posted by
Alberel
Assuming FFXIV is following FFIII's concept of dark and light (and Elidibus' reference to equilibrium suggests they are) the warriors of darkness are born of a separate crystal of darkness. You do not choose the light or dark, and they do not represent good or evil. The warriors of darkness are essentially our equivalent fighting on the other side of the balance. It would be impossible for players to ever switch to being warriors of darkness because we are born of light, not darkness. It's simply an impossibility within the entire concept of the duality.
Unless SE pulls something like the WoL dies near the end of 3.X era and loses all his Light due to his or her death. At that point the WoL may be given a choice to return as a WoL once again or as a WoD.
Assuming SE may give players a Choice to start a new character in 2.0 era or 4.0 era in 4.0 release. 
After all once a WoL dies..
SPOILER!
All the light is released from the WoL just like what happened to Ysayle