NONREFUNDABLE, nonexchangeable. Those two words carry severe financial penalties for many airline passengers when travel plans change but tickets - often bought weeks or even months in advance - cannot. Only when a traveler can prove a medical emergency, certified by a doctor's letter, will most carriers refund the price of low-fare tickets. Otherwise, cancellations or last-minute changes can involve buying a new ticket or forfeiting up to 50 percent of the ticket price.Now Midway Airlines is taking a more passenger-friendly approach with a welcome new policy. All discounted excursion fares on the Chicago-based carrier are fully refundable, except for a 10 percent service charge on changes or cancellations. That fee will cover travel agents' commissions on canceled tickets. In addition, Midway's lowest fares now require only a seven-day advance purchase, rather than 30 days. In the past, no-shows - customers who reserve seats but fail to travel - cost airlines considerable lost revenue for seats that went unfilled at the last minute. To protect themselves, carriers instituted rigid rules and stiff penalties on reduced fares. These policies fill seats, but they also deter customers who are unable or unwilling to lose money if their plans change. Other carriers should follow Midway's lead in reducing penalties. If they remain unwilling to do that, perhaps they will at least consider policies that are less punitive than those now in effect. Why not, for example, allow travelers a partial credit on the price of canceled tickets, good toward future travel on the same airline? This would foster goodwill and build customer loyalty. After all, if full-fare business travelers are the bread and butter of the airline industry, budget-conscious vacationers are surely the jam and honey, indispensable in sweetening carriers' revenues. Meanwhile, for those flying Midway's routes, the skies have just become friendlier than they've been for a long time.