You could try delaying your flight for around 10 to 15 years, and embarking on your career plan. If you are succesful (and the airlines haven't gone out of business by giving their premium class products away,) you may be lucky enough to become a pilot with SQ. That would give you a very good chance of a free upgrade.
Failing that you might find yourself on an overbooked economy flight where the airline is looking to upgrade somebody who is suitably dressed and assuming there are no quality frequent flyers who the airline feels deserves the benefit. A bit of a long shot, but it happens.
Failing that and slightly more in the realms of reality, you could request or purchase an extra leg room economy seat in the vicinity of an exit etc. There may or not be an extra charge for this. The airline may be sympathetic to your injury and comply with such a request on that basis. However it is worth remembering that exit seats should only be occupied by able bodied passengers and an injury plea might not help your case in this regard.
Lastly you can simply ask for an upgrade at check-in. The check-in agent will not look in the least bit surprised and simply provide you with one of the stock answers they are trained to provide to this very common question.