I believe that if you move the commander's seniority up by 4, you will be able to promote him. If there are leaders of the same rank, but higher seniority (lower number), then there will be NM & VP penalties for promoting junior generals over senior generals.
'Nous voilà, Lafayette'
Colonel C.E. Stanton, aide to A.E.F. commander John 'Black Jack' Pershing, upon the landing of the first US troops in France 1917