I'm not sure if this is what you're going for, but most of the higher leadership of the Confederate army were formerly members of the United States army.

Robert E. Lee was a West Point graduate and long serving officer who, because he didn't want to fight against Virginians, essentially committed treason (or at least broke his officer's oath) by joining the Confederacy.

You can find a list of other Confederate generals who graduated from West Point here.

Are you asking about enlisted soldiers or officers that defected during battles after the war was underway? Many southern officers that were in career service in the in the US Army and had served in the Mexican-American War resigned their commissions to officer for the Confederacy.