Earning airline frequent flier miles is getting harder and more expensive. United Airlines is the latest carrier to be making big changes to its frequent flier program. The airline will soon give more miles to customers that have one of the carrier's co-branded credit cards over those that don't.The changes mark the biggest shakeup in the program in more than a decade. Starting in April, customers without one of the credit cards will earn fewer miles than those who have one, and those with the card are goin ...