(a) Generally. Should a Judge become unavailable to the Commission, the proceedings assigned to him shall be reassigned to a substitute Judge.

(b) Substitution following a hearing. The substitute Judge may render a decision based upon the existing record, provided the parties are notified of his intent and they are given an opportunity to object. An objection to the Judge rendering a decision based upon the existing record shall be filed within 10 days following receipt of the Judge's notice, or the objection shall be deemed to be waived. An objection shall be founded upon a showing of a need for the resolution of conflicting material testimony requiring credibility determinations. Upon good cause shown the Judge may order a further hearing on the merits, which shall be limited, so far as practicable, to the testimony in dispute.