If it is a genuine package holiday put together by the agent, then as the 'principal' it is their responsibility to put the package back together again, if one of their suppliers, in this case CO, have let them down. Clearly they can't magic a flight out of thin air, but they could re book you on the same day through a different airport or rebook on a different day at the same price. If you choose to change days you pay, it works the same way if they change the day.