ALTER TABLE vehicleassignement
    ADD COLUMN startTimestamp        TIMESTAMP WITH TIME ZONE,
    ADD COLUMN endTimestamp          TIMESTAMP WITH TIME ZONE;

UPDATE vehicleassignement SET (startTimestamp, endTimestamp) =
    (SELECT startTimestamp, endTimestamp FROM event
     WHERE event.id = vehicleassignement.eventId);

ALTER TABLE vehicleassignement
    ALTER COLUMN startTimestamp SET NOT NULL,
    ALTER COLUMN endTimestamp SET NOT NULL,
    DROP COLUMN startTime,
    DROP COLUMN endTime;