United allows cabin crew to sell "upgrades" to Economy Plus (not technically an upgrade, since Economy Plus is still the economy class, just with more legroom) from their handheld devices.
Source :
If passengers want to upgrade to economy plus during boarding, for example, Bingochea can easily accommodate the request using a proprietary app on his handheld device called Link.
The devices they use take credit cards, same way they sell food and drinks.
I'm not sure whether they can do real upgrades to other cabin classes though.
travel.stackexchange.com/users/13650/Zach Lipton
The other day Lufthansa was selling Premium Economy upgrades after the door was closed.
The price was ridiculous, though, and no one went for it.
travel.stackexchange.com/users/13971/Hilmar
Edelweiss allows you to upgrade from Economy to Business after boarding the plane.
At least for short-haul flights, subject to seat availability.
Payment can be done through credit card or debit card.
This even works, if catering does not allow for the +1 on the manifest, which it normally does.
I don't know if it would work on long-haul flights, since I never found myself in that situation.
Couldn't find the official confirmation on this.
But I've done it a couple of times for flights between ZRH and CTA, where buying P/Z/C/D tickets 2-3
hrs beforehand was not possible anymore.
Or if I wanted to upgrade a fellow traveler to fly with me.
In the past other airlines allowed me to pay a fixed fee on board to upgrade, but I haven't really flown with most of them in quite a while.
travel.stackexchange.com/users/20159/Moreaki
Air France/KLM allows upgrades on board by speaking to a flight attendant: UPGRADE LAST MINUTE ON BOARD
You can even upgrade your cabin after boarding, subject to availability.
Eligible airlines: Air France and KLM marketed and operated flights Available payment methods: Miles or just cash
How to request: on board
travel.stackexchange.com/users/70877/jdouglas
Scoot (Singapore Airlines' low-cost subsidiary) used to offer paid in-flight upgrades , but this was suspended in June 2020 due to the COVID pandemic.
travel.stackexchange.com/users/1893/lambshaanxy
You can always request an upgrade to a higher class before you board, you can use your miles or pay and get your ticket upgraded to business/premium class then you will be issued a new boarding pass.  However, are there any airlines that provide that  after  boarding? like you call a flight attendant, request an upgrade, pay for it somehow, and then get upgraded on-spot?  Please note, I am talking about  paid  upgrades.
Are there airlines that offer upgrades on-board?
travel.stackexchange.com
<airlines><upgrades>
travel.stackexchange.com/questions/159011
travel.stackexchange.com/users/10051/Nean Der Thal
Upgrades are sometimes available after boarding on some airlines, either for economy seats with increased legroom or cabin upgrades.  Examples include United, Lufthansa, Edelweiss, Air France/KLM, and Scoot. Some upgrade offerings may have been suspended due to the COVID-19 pandemic. Some airlines only offer changes to premium economy services, though others allow you to upgrade when boarding or even after you've boarded.
Certain airlines offer changes to premium Economy services, though it is questionable whether these qualify as real upgrades. Numerous airlines offer upgrades upon and even after boarding.