Here, we'll talk about how you can - and can't - use your miles to upgrade to a better seat.
Using Points For Upgrades
One of the most common questions I get is some version of "I've got a bunch of points, can I use them to get an upgrade on a cash ticket?"
Unfortunately, the answer to this question is almost always no. On most airlines, miles can only be used to upgrade "full fare" tickets; that is, the fully-flexible, fully-expensive ticket that may have cost as much as business class or more in the first place.
If you *do* find yourself on one of those tickets - the fare class will usually say "Economy - Y" or "Economy - B" then you may be able to upgrade with miles if there's award space available. That is, if you can find an option to book a brand-new award in business, on the flight you want, and you're booked in a "Y" fare, then you can very likely use miles to upgrade.
Using Miles To Bid For Upgrades
Air Canada has had an option for many years in which they "auction off" unsold business-class seats in the day or two before departure. While this has previously only been available to cash bids, in 2020 they launched a new option to bid with Aeroplan miles instead of cash.
This process works pretty much like the existing cash-bid system; when you log into your reservation, you may be offered the option to submit a bid; the website will set a minimum and maximum bid, and seats will be awarded to the highest bidders, usually until the remaining premium seats are all full.
In a recent example, a flight from San Francisco to Vancouver - about two hours long, in the "recliner style" business seats - offered this upgrade with a minimum cash bid of $350 CAD, or a minimum miles bid of 35,000. So, they're essentially treating 1 mile like it's worth 1 cent, which I consider a relatively low value, but if you've got the miles to burn, it may be a worthwhile option.
If You've Booked An Award, But Some Segments Ended Up In A Lower Class
Let's say you've booked a business-class award ticket from Toronto to Tokyo to Bangkok, and by bad luck, business class was only available on the Toronto-Tokyo leg.
The bad news is that you'll still be charged the same number of miles as if the whole trip was in business. The good news is, on some award tickets, if business-class award space opens up later on for the Tokyo-Bangkok leg, you can simply call in and get bumped up for free!
Or, at least, "nearly free", as some countries do charge additional taxes on premium-cabin passengers, which might hit you for an extra few dollars, but that's generally a pretty good trade to upgrade a long flight! In particular, beware of France and the UK, both of which can charge up to several hundred dollars per passenger in additional taxes on long-haul Business or First-class passengers.
Regardless of destination, it's definitely worth checking on this at 13-14 days prior to departure, and again 3-4 days out. It doesn't always work, but it sure is nice when it does!
When I book award tickets for clients, in this situation I set tracking software to check this every 12 hours, often for months at a time; the minute additional space opens up, I'll be in touch to let you know that the airline is ready to process your free upgrade!