Complex sentence: The 1st plenum saw the "revolutionary generation" retire from their executive posts, surrendering them to the new generation of Kim Jong-il; however, they still controlled the highest organs of power: the Presidium and the Politburo.
Split into simpler sentences: The 1st plenum saw the "revolutionary generation" retire from their executive posts, giving them up to the new generation of Kim Jong-il. However, the revolutionary generation still controlled the highest organs of power, the Presidium and the Politburo.