By Amy E. Feldman

PHILADELPHIA (CBS) – Changes are in the air for frequent flyer programs in 2015.

Nothing is more frustrating than trying to use your frequent flyer airline miles, except maybe everything else associated with airline travel. But now it’s going to be more confusing too.

It used to be that you earned miles based on, you know, miles. But now, Delta and United have announced that upcoming changes will mean you’ll earn miles based on dollars. So whether you paid $200 for a ticket or $1000 for a ticket to the same destination didn’t used to matter – you got the miles associated with the trip. But now, you’ll earn fewer miles if you paid fewer dollars.

That means that if you are the type of flyer who shops for discount flights, you can expect to earn fewer frequent flyer miles.

But, if you already signed up with one airline because of its program, and now find out that new rules will disadvantage you, you want to know: can they do that?

Once you’ve signed up with a plan (presumably to the exclusion of other airline plans), can your plan legally change its rules mid (jet) stream? Yep.

When you sign up for a frequent flyer club, you agree to the terms of service, which basically say that the airlines can set the terms they choose, including the ability to change or even cancel their plans with or without notice. So re-read your plan to see what you’re really earning. You may see that the skies are even less friendly than you thought.