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*).

For the OP: if it's less than 24 hours since you bought the ticket, just cancel it, then buy the one you meant to. United allows this without penalty: http://www.united.com/page/article/0,,1510,00.html