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

(b)  Each rehearing shall take place before a court-martial composed of members not members of the court-martial that first heard the case. Upon a rehearing, the accused may not be tried for any offense of which he or she 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.