i've looked at the lowest fares in some of my usual markets, and i'm seeing this everywhere:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">TICKET HAS NO VALUE UNLESS PASSENGER CANCELS TICKETED FLIGHT RESERVATIONS PRIOR TO TICKETED DEPARTURE TIME. CHANGES CHARGE USD 100.00 FOR REISSUE. NOTE - ITINERARIES MAY BE REBOOKED UP TO ONE YEAR FROM ORIGINAL TICKET ISSUE DATE PROVIDED PASSENGER CANCELS THE TICKETED FLIGHT RESERVATION PRIOR TO TICKETED DEPARTURE TIME. </font>
i'm going on the assumption that published fares are changeable until i see a couterexample. (that said, i'll be reading fare rules before i book, looking for counterexamples... :-) )