United MileagePlus has a little-published feature of their rewards program: waitlisting for a higher class of service. This only applies to flights operated by United and booked using United Mileage Plus miles. Partner flights cannot be waitlisted and you are stuck in the class that you book.
I will use my own booking mentioned in the previous post as an example. I am booked to fly on United Airlines from Jacksonville, Florida to Singapore with stops in Houston, Texas and San Francisco, California.
Right now I am confirmed in economy class on the JAX-IAH and IAH-SFO segments. My flight SFO-SIN is in business class and was confirmed at the time of booking. In this case United allows me to waitlist for domestic first class on JAX-IAH and IAH-SFO since I “paid” for business class in terms of miles redeemed for the entire journey. If my waitlist clears it will cost me nothing above what I have already paid!
Getting on the waitlist is not an obvious process at all. I had to do some Googling to find the information on the United Airlines website and a couple of other blog posts. I will save you the trouble: Just reach out to United on Twitter @united and ask them to place you on the waitlist. Calling will get you a mixed result based on how well trained the phone representative is. Trust me: just use Twitter!
The process of getting the waitlist done via Twitter was fast and easy. I simply wrote a public tweet @united asking for help and they responded (below) within a few minutes asking me to DM (direct private message) my details to them. I had to send my them confirmation number, name as shown on the ticket, and email address.

All told it took about 30 minutes (mostly waiting time for replies) to get this done efficiently via Twitter and minimal effort. United’s reply is shown below after I provided the info they requested via Direct Message.

I then logged into my United MileagePlus account and found the waitlist for both domestic flights (shown below). Note I had to use a computer and web browser to see this. Waitlist does not show on the United app for some reason!

As you can see in the above screen grab I am already confirmed in Polaris business class on the long haul flight to Singapore. Below that shows my waitlisted requests for domestic United First class. I left out my confirmed economy segments in the screen grab for clarity.
There is a chance the waitlist could clear long before departure. If so I will be notified by United via email and my itinerary will update to reflect the confirmed “upgrade”. If it does not clear at least 24 hours before departure I am placed (most likely) at the top of the upgrade list at the airport. If a seat is available I will be upgraded from economy class to first class.
Worst case scenario I will have to fly in economy class if the waitlist does not clear for some reason. Though, from my own experience and other travelers’ experiences, it is quite likely I will get upgraded. Regardless, I am happy with what I booked and domestic first class is just a bonus to me!
Tip: United actually allow waitlisting on trips booked entirely in economy class using MileagePlus miles too! All segments must be on United operated flights. Partner flights cannot be waitlisted for upgrades. You do have to pay the business (or first class) price in miles at the time of booking. Again I recommend using Twitter if this is something you want to do.
If none of the waitlist upgrades do not clear you are refunded the difference in miles if you fly your originally ticketed class of service. I have personally never tried this method as I prefer to at least have the long haul flight confirmed in business class if I am fronting the miles for it. For those with a large balance of miles this may be useful if you have fixed travel dates.
Overall the waitlist “secret” feature of MileagePlus is a great way to book an acceptable itinerary and possibly get upgraded as an added bonus. With future travel uncertain and light passenger loads I feel that upgrades are very likely!