You won't have any trouble upgrading to first class at the airport if you're a frequent flyer. All you have to do now is use frequent flyer miles to upgrade. This will also help you move to first-class if you fly at least 25,000 miles every year. In these situations, elite frequent flier members gain the most.
It should be noted, however, that getting upgraded to first class is not always straightforward. You can still inquire at the ticket desk if any first-class tickets are available on the aircraft if necessary.
If the plane isn't fully booked, the airline may be able to offer you an upgrade for a far lower price than you would have paid if you had purchased a first-class ticket in advance.