(a) If the convening authority disapproves the findings and sentence of a court-martial, he may, except where there is lack of sufficient evidence in the record to support the findings, order a rehearing, in which case he shall state the reasons for disapproval. If he disapproves the findings and sentence and does not order a rehearing, he shall dismiss the charges.

(b) Every rehearing shall take place before a court-martial composed of members who were not members of the court-martial which first heard the case. Upon the rehearing the accused shall not be tried for any offense of which he was found not guilty by the first court-martial; and no sentence in excess of or more severe than the original sentence shall be imposed unless the sentence is based upon a finding of guilty of an offense not considered upon the merits in the original proceedings or unless the sentence prescribed for the offense is mandatory.