This is what the update notes say:
Fixed Key
Players will be able to map functions to previously fixed keys.
* The following keys will remain fixed.

* 0 -9
* /
* ScrollLock
* PrintScreen
* Pause
Why are these keys still fixed? Let me guess: the game is coded in such a way that messing with these would somehow break the game completely? I cannot fathom why else they would leave these fixed especially considering how being able to change the action bar buttons is pretty much why we've been asking for changes like this.