Looking at Earth (war in Syria, or Libya before that), modern rebellions seem to be often based on having high level generals defect from the government forces to the Rebels.
But everyone in Star Wars rebellion who was an Imperial before, was a civilian (Senators Mon Mothma, Leia Organa, Bail Organa).
Did high ranking Imperial officers join the Rebellion? Or were they really 100% loyal to the Emperor? The only one I could think of, General Dodonna, wasn't an officer when he joined (he had retired from the Imperial military). Admiral Ackbar was a slave to Tarkin.