Shouldn't be a problem. Just swap boarding passes. If you're both the same sex, no need to inform the crew. If different sexes, you might let the crew know.
Where it's not allowed is if you have say paid for a child fare in business class, and you want to swap an adult from economy into the business class seat.
I've actually done it a few times. My BP was J and the other pax was Y. Crew were fine but said it would be a one-way swap. No 50/50 changes half way though etc.
Done this on SQ no problem. Very much asked cabin crews permission though - I swapped with my grandmother in law so might be pretty obvious if they looked at the date of birth!
The Frequent Flyer Concierge team takes the hard work out of finding reward seat availability. Using their expert knowledge and specialised tools, they'll help you book a great trip that maximises the value for your points.
From just $6 we'll remove all advertisements so that you can enjoy a cleaner and uninterupted viewing experience.And you'll be supporting us so that we can continue to provide this valuable resource :)