Originally Posted by someotherguy

The no-fee exchange for a higher-priced ticket depends on the fare rules and is usually seen only on international fares. For example, here's what the rules say for WKWSN8E (IAD-FRA): "THE FULL AMOUNT OF THE FARE PAID MAY BE APPLIED AS A CREDIT TOWARDS PAYMENT OF ANY HIGHER APPLICABLE FARE TYPE". This allows you to buy up to, say, an H fare without penalty, but the way I read it, you can't change flights/routings/dates within the same fare class, even if the new fare is higher (because it specifies higher fare *type*)