oversold business class free upgrade

If a flight is oversold on popular routes like london LHR to los angeles LAX/new york JFK and there are open seats in first or business class, the airline needs to upgrade coach class passengers. In this case you may have a chance for a free upgrade.
Keep in mind that if you call the airline up and ask them if the flight is overbooked, they are required to answer the question

there are two groups with more potential for a courtesy upgrade than others:
1. top tier members of an airline’s frequent flyer program;
such as Delta SkyMiles Diamond, Platinum or Gold Medallion member
Cathay Pacific Marco Polo Club Gold and Diamond.
British Airways Executive Club Silver and Gold
Mileage Plus elite status on United airlines.
Star Alliance Gold status
2. passengers traveling on a full fare economy class ticket.

This remind me of a KLM flight a few months ago, I was flying Washington Dulles to Amsterdam and asked at the gate at IAD if the flight was oversold and on both occasions the answer was "no" . I then asked if I could use my miles to upgrade my ass to a J class seat but again the answer was "no" as I was travelling on a W class ticket. Of course I had the possibility to pay more than $600 for the IAD-AMS upgrade but refused the offer, and I found later on board that the flight was 100% full in Y and Y+ and that they had to give some free upgrades to some lucky passengers

By becoming a frequent flyer in an airline, there are more chances that you would get free upgrade from coach to business class. You must also earn a valid upgrade certificate and try keeping your frequent flyer ranking at the highest possible level. There would be an increase in your potential to get courtesy upgrades if you maintain as a top level frequent flyer and travel on oversold flights of certain airlines.

Sometimes the travel upgrades in business class flights would depend upon the company that you keep during the flight. If you are not a top tier member of frequent flyer program, you don't have to lose hope. You must try to keep the company of a top tier member and this would help you in a favorable way. You must consider yourself lucky if you get a chance to travel on an undersold or full fare flight. Also the empty flights show chances of getting a business class ticket as weight and balance are the major concerning areas in an airline. If the flight is empty, some fortunate passengers may get an upgrade from economy class.