I think this is a genuinely grey area. If the employers policy is a 'business over X hours' and no requirement to work on arrival, then its perhaps okay to do it, if the individual is fit for duty on arrival. If business is approved because of the need to do work on arrival, swapping seats to a partner would be a grey area - you've been given a benefit to enable you to do your job, and you are choosing to instead effectively self fund travel and give the benefit to someone else instead. I can see many company HR policies taking a very dim view of this.

An example I am personally aware of where this sort of abuse fails the 'sniff test' - many (and I mean many) years ago, a serving British Army General was tasked to go to Australia for business reasons. He took his Military Assistant (MA) - a much younger and more junior officer effectively functioning as his Chief of Staff with him. MOD policy at the time was that travel to Australia was staff travelled business class, with work from the next day due to the length of travel - I don't know what it is now.

On boarding the plane, the General said to the MA, as you can see MA, Mrs General is on the flight too, and she's booked into economy. I'd be very grateful if you'd please swap places with her and I'll see you in Oz. The result was that the General had purchased an economy ticket for his wife, and strongly encouraged his MA to swap seats to economy and gave his wife a free business class trip to Australia funded by the taxpayer. This is a long time ago and all personalities have moved on (and in some cases passed on), so I feel comfortable sharing it, but it also highlights the potential for abuse of the system of seat swapping classes.

From my own experience, some years later I also had to go to Australia for work and was able to legitimately tag some leave on at the end - I flew my wife out to join me after the work trip ended and then we flew back together. We took the view that as the office was paying one of our seats, it was essentially a 'buy one get one free deal', so we paid for her to fly business with me as the cost was still overall half the cost of us doing it purely for leave. If in doubt, fly your partner in the same class as you're in, or accept you're in different classes for the trip.