Originally Posted by CO.com website
Mileage Credit for Cancelled Flights
When you are ticketed in advance and your flight is cancelled on the day of departure, we may re-accommodate you on another flight. If you are re-accommodated on Continental Airlines or a OnePass airline partner, you will receive mileage credit for the new flight(s) only.
If you are involuntarily re-accommodated on a non-OnePass airline partner flight, you may request mileage credit for your original flight itinerary. Send a written request for mileage credit to the OnePass Service Center. Explain the circumstances of the flight, and include a copy of your original passenger receipt as well as the boarding pass from the flight flown.
Exceptions include flights cancelled by uncontrollable circumstances (e.g., work stoppage or weather). In these situations members will not earn miles.
Though it says you only receive mileage for the new flight, there are tales on here of people getting credit for the original flight. This is important at times if the original flight would have yielded more RDM, EQM, or EQP. You still only get credit for one of the flights... no double-dipping when they put you on a partner airline.