(a) If an ALJ cannot perform the duties described in § 20.202 or otherwise becomes unavailable, the Chief ALJ shall designate a successor.

(b) If a hearing has commenced and the assigned ALJ cannot proceed with it, a successor ALJ may. The successor ALJ may, at the request of a party, recall any witness whose testimony is material and disputed, and who is available to testify again without undue burden. The successor ALJ may, within his or her discretion, recall any other witness.