I would write customer service and ask for miles needed. We called to let CO know we would not make a flight and were given seats on the next flight out without problems at ticketing. The agents at ticketing and at the gate said we were confirmed for our return. Day of return, our flight and flight before ours was oversold, and agent put people from earlier flight on our flight, but said we had no reservation. A phone call and follow up letter to CO, we were sent vouchers and a fruit basket.