To answer the original question (not whether it makes any sense to do it), the easiest way is to book the lowest fare class all the way through up to but not including purchase. At the last possible moment, hit the cancel button. You'll get a pop up window with your booking locater (held for 3 Hours, IIRC) and the telephone number to co.com support. Call, select the option for co.com support (3 the last time I called), tell them the booking locater and what you're trying to do. They can change the fare class of any segment (assuming it's available) and the whole thing remains a co.com purchase.