Who doesn't like a flight upgrade? It turns out there's one new gateway to upgrades that you should know about if you have an American Express card. The card company announced last week that Membership Rewards points can now be used to bid for upgrades on flight reservations that have already been made.

American Express is working with a company called Plusgrade to make it happen. More than 21 airlines have already joined in to allow their customers to take advantage of this perk. The list includes big carriers like Singapore Airlines, Qantas and Air Canada. Unfortunately, no carriers based in the United States have decided to join as of right now.

How can travelers begin to take advantage of this new perk to claim upgrades? The good news is that savvy point seekers can potentially get upgrades without paying a cent. The bad news is that nothing is actually guaranteed. A processing tool has been embedded on the American Express website that makes it easy to bid.

Eligible travelers with Membership Reward points simply need to select their preferred partner carrier and enter their reservation details. This will allow them to check for eligibility. Passengers with eligible reservations will then be able to bid on an upgrade using points or a combination of points and cash. However, it doesn't end there. Customers actually need to be prepared for a bit of a waiting period. That's because the Plusgrade tool pitches your upgrade bid to your carrier. The carrier then has five days to either accept or reject your bid.

Scott Dylan is a contributing writer at GET.com and has been to (almost) every country in North, Central and South America with nothing more than a backpack, a laptop and the desire to explore. He speaks Spanish fluently and has logged enough time in planes, trains, rideshares, buses, taxis and rickshaws to know how to rack up rewards and points to get anywhere his heart desires for pennies on the dollar. Email: scott.dylan@get.com.