The nation's airlines have rolled backa recent harsh rule on nonrefundable ticketsthat basically told flyers to use them orlose them. If a traveler didn't use a nonrefundableticket for the designated flight,the ticket was worthless and couldn't beexchanged for a different flight. Travelersmust still be wary, however, since carriershave a whole range of regulations aboutexchanges. Some give you a year toexchange the ticket for a different flight,with the countdown starting from thedate of the first flight on your itinerary.Other airlines start the clock on the dateyou bought the original ticket. Some offerexchanges for tickets bought on discountWeb sites; others don't. And most will notexchange tickets bought on bidding siteslike Priceline.com. Check with yourairline for specific exchange rules.