http://www.dot.gov/airconsumer/codeshare-disclosure

http://www.dot.gov/sites/dot.dev/fil...odeshare.8.pdf

http://www.ecfr.gov/cgi-bin/text-idx...57_main_02.tpl

From the above, DOT requires that airlines provide clear notice to customers of the airline operating each flight included in a ticket purchase transaction. Since clear notice prior to purchase is required, it seems to me that it follows that if the operating carrier is changed (for example mainline to connection), the ticket becomes fully refundable (to original form of payment, not a voucher) at the customer's option.