I believe IIRC that Cotaez is the only character from C:GK that can change all his powers out for BRB powers. An Inquisitor can also change out his one power and probably both should a Daemonblade make him level 2.
From the Grey Knights FAQ:
A Grey Knights Librarian may use the psychic disciplines found in the Warhammer 40,000 rulebook, instead of those in Codex: Grey Knights. If he does so, generate a number of psychic powers equal to his Mastery Level from the Divination, Pyromancy or Telekinesis disciplines (in any combination) before armies are deployed. If he does so he may not purchase additional powers. Hammerhand cannot be exchanged in this manner, but is instead retained in addition to the powers purchased or generated by the Librarian.
An Inquisitor with Mastery Level 1 can exchange his psychic power for a single roll on either the Divination, Pyromancy or Telekinesis disciplines. Coteaz can generate two powers from either the Divination, Pyromancy or Telekinesis disciplines (in any combination) in place of those listed on his profile. In the case of Coteaz, unlike the Grey Knights Librarian, the Hammerhand power listed on his profile is replaced by one of the powers generated as normal.