Honestly I don't think it is arrogance but incompetence or inefficient ticketing operations (even in FRA). The OP was willing to pay the fare difference and not asking for some cheapo way to sneak into a higher class of travel so not being able to collect that money is just ridiculous from a business perspective. I think they need to do something about their ticketing system and fix this. If someone walks up at the airport counter and wants to drop a few thousand SGD to get a better seat I want to be able to take his money and sell him the seat if it is available.

... and knowing some guys working at Temasek (SQ's majority owner) they would think the same.