I fly by myself quite frequently and have been upgraded to first class many times — always without asking. I doubt I would have received any of these upgrades if I hadn’t been in a wheelchair. I also think had I been traveling with someone then it’s likely I would not have gotten upgraded — so take my experiences with a grain of salt if you’re flying with a companion. And of course by no means does this happen the majority of the time. It’s usually one in every ten flights or so.
Sometimes I think it all starts with my initial choice/assignment of seat. I have found that I tend to get an upgrade when the agent has to move my seat forward to better accommodate boarding/deplaning. For example, one time I had selected row 12 but somehow got bumped to row 46 at the time of check in. The gate agent knew this would be a pain in the ass to deal with when getting me on board via the aisle wheelchair and tried to move me up. Fortunately for me, the closest available seat was in the first class cabin! By no means do I recommend specifically selecting a seat in the back of the plane just to encourage this sort of upgrade. If you can get a seat closer to the front, take it whenever possible.
Usually at the airport I am extremely cordial at the check-in counter. You never know when that agent will end up at your gate as well, essentially with the power to put you in a nice first class seat. Once at the gate, I usually go to the counter and say something like “I’m sorry to be so high maintenance, but could you give me a gate claim tag for m wheelchair?” I am extremely appreciative and nice. Then I also just remind them that I will need an aisle chair. At this point they usually grab my ticket to see where I am sitting. On a few occasions this is the point where my seat has been upgraded.
Lastly, I have benefitted from several last second first class upgrades. These occur when they have me strapped into the aisle chair and all ready to get on board. Sometimes it will be much easier for them or faster (if we’re already running late) to dump me in a first class seat instead of take me all the way back to my regular seat. I can think of at least three times this has happened.
Of course you can try to get bumped to first class by asking, but the times I have done this I have been met with reluctance. Also, airlines have started selling last minute upgrades on flights at check-in time if they need to fill up first class. So the stock response when you inquire upon check in is usually, “We are offering first class upgrades right now for $250.” If I am feeling particularly weary and actually WOULD pay for a first class upgrade, then I usually do ask if I can purchase one when checking in.
So sometimes it’s better to just leave it up to circumstance, fate, or luck and see what happens. Happy travels!