I think the answer has been yes in the past according to posts I had read, but I tried four agents last month and none would do it for me, all of them insisting that I needed to cancel and rebook, with no guarantee the existing flights would go back into reward inventory. Fortunately there was an alternative to a different airport which just about worked so I booked that instead, using the 2-4-1, then cancelled the original flights, not expecting them to go back into reward inventory as there were no reward flights in business or first for a month either side. Much to my amazement, they did though, so I cancelled our replacement flights and rebooked the original ones using the 2-4-1. With the benefit of hindsight I should just have cancelled the original ones to see what happened, but at least the extra £50 cancellation fee gave me peace of mind that we had something sorted. So all in all £100 to add the 2-4-1, which is a shame but not the end of the world.