If the convening authority disapproves the finding 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 such a case he shall state the reasons for disapproval. If he disapproves the findings and sentence and does not order a hearing, he shall dismiss the charges.
Each rehearing shall take place before a court-martial composed of members not members of the court-martial which first heard the case. Upon a rehearing the accused may 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 may 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.