(a) If the convening authority disapproves the finding and sentence of a court-martial, the authority, unless there is lack of sufficient evidence in the record to support the findings, may order a rehearing. The authority shall state the reasons for a disapproval. If the authority disapproves the findings and sentence and does not order a rehearing, the authority shall dismiss the charges.
(b) Each rehearing must take place before a court-martial composed of members who were not members of the court-martial that first heard the case. On a rehearing the accused may not be tried for any offense of which the accused was found not guilty by the first court-martial, and sentence in excess of or more severe than the original sentence may not be imposed, unless the sentence is based on a finding of guilty of an offense not considered on the merits in the original proceedings, or unless the sentence prescribed for the offense is mandatory.