Why it's easy to confirm upgrades on these United 787 routes

United Airlines is the only U.S. carrier to operate all three Boeing 787 Dreamliner variants. At the moment, however, United's flying four different configurations of the plane, as it works through the process of updating its fleet of 12 Boeing 787-8s.

As a result, United Dreamliner passengers could end up with any of the following arrangements, with seat counts broken down by Polaris/Premium Plus/Economy Plus/Economy:

• Boeing 787-8 (28/21/36/158)
• Boeing 787-8 (36/0/70/113)
• Boeing 787-9 (48/0/88/116)
• Boeing 787-10 (44/21/54/199)

The airline has also begun retrofitting its first 787-9, which, when complete, will add a fifth configuration to the fleet:

• Boeing 787-9 (48/21/39/149)

As you may have noticed, retrofitted 787-8s only offer 28 business seats, in a 1-2-1 arrangement, compared with the 36 flying in the older 2-2-2 configuration today. Since United typically doesn't assign aircraft to specific flights until a couple days before departure, the airline is simply "blocking" those eight extra business seats, to avoid having to downgrade Polaris passengers to economy if the refreshed aircraft ends up operating their flight.

Then, assuming the flight ends up being operated by the older 36-seater, a gate agent will clear eight economy passengers into business class just before departure — assuming you don't end up with the refreshed plane, an upgrade is essentially guaranteed for the eight flyers at the top of the waitlist. Customers redeeming PlusPoints and upgrade awards will clear in order of elite status and fare.

So where can you find the 787-8 Dreamliner? Currently, United flies the aircraft on the following international routes:

• Denver (DEN)-Frankfurt (FRA)
• Denver (DEN)-London Heathrow (LHR)
• Denver (DEN)-Tokyo Narita (NRT)
• San Francisco (SFO)-Chengdu (CTU)
• San Francisco (SFO)-Osaka (KIX)
• San Francisco (SFO)-Paris (CDG)
• San Francisco (SFO)-Tahiti (PPT)
• Washington, D.C. (IAD)-Beijing (PEK)
• Washington, D.C. (IAD)-Paris (CDG)

In addition to select domestic flights between the following cities:

• Denver (DEN)-Washington, D.C. (IAD)
• Denver (DEN)-San Francisco (SFO)
• San Francisco (SFO)-Washington, D.C. (IAD)

If you see a seat map that resembles what appears below, you've landed the new Polaris configuration — great news, assuming you're already booked in biz.

These planes have 21 premium-economy seats, too, and they're being sold as Economy Plus for the time being, which means elite flyers can select them for free — if you see this arrangement appear and you're booked in coach, be sure to grab a seat in row 20, 21 or 22.

Meanwhile, if you see the map below, your flight is assigned to an older 36-biz-seat plane. Note that this map appears by default — you'll want to pay attention to the flight status page a day or two prior to departure.

Of course, as more 28-seaters enter service, it'll be increasingly difficult to score a gate upgrade, since there will be fewer aircraft with those extra eight seats.

For the enthusiasts in the room, you can track United's retrofitted plane by its tail number, too — N27908. Meanwhile, N26906 and N26909 are both undergoing retrofits in China, and should re-enter service with the new seats, soon.

Featured photo by Zach Honig/The Points Guy