UMMMM...Yes they were..
Respectfully, you are incorrect. There never was any contract between Delta and Comair as it was a subsidiary. Therefore, rates, block hours, bases etc could be changed at will without violating a contract or without requiring the renogotiation of a contract.
Rates at which Comair was reimbursed on the books could be changed at will also, without notice. This is why, in spite of record profits, Comair was taken into bankruptcy (and emerged from bankruptcy) simultaneously with Delta.