Capetonian,
I think you have raised a good point regarding the class of ticket being purchased. What is the motivation of the passenger travelling with a full service airline?
Is it to get the most amount of service for the least amount of money? - the freloader argument - I'll buy a cheap ticket and if I need a change I'll busk it with my club card and , if that fails, pay the extra. or,
I want that route, that timetable but the full ticket price - exceeds its true value. Why should I pay that? - the frugal flyer
Whichever the circumstance - the Frequent flyer programmes can assist airlines in deciding how to treat their customer base - if the process allows. If it's the busker, then a one off - OKay sir /madam but....next time we'll need to make a full charge unless you have paid for a flexible ticket.
For the frugal flyer, assuming that he has earned some tier points in the past, is this not a great opportunity to demonstrate the benefits of paying a little bit extra in the future?
The result of saying sorry sir/madam, to either of the above is likley to be the same - a perception of poor customer service and no little desire to repeat the experience. In both of the above cases the airline has right on its side but will be the eventual user.
Crepello perhaps suggests that in some way, by saying no to a customer, you will develop and command respect - I'm sorry, but on that count you are most probably wrong!