...protested that due to lack of space on the busy flight there was insufficient room for him and so he should be upgraded to J.
Cute move but... I wouldn't try that now. If you need two seats, you pay for two seats.
And I wouldn't try a DIY upgrade on the plane. Also new Gate Systems (eg. ALTEA) have an on-load priority list. The system decides according to various criteria who will be upgraded and who will be off-loaded. Over-riding the system is difficult and very "noisy" and the back-doors of previous systems are bolted shut. On the aircraft, the senior cabin crew are sent electronic passenger lists on their tablet computers and are charged with ensuring passengers are sat where they are meant to be.