Originally Posted by Ldnn1

Are you using a 241?

If not then the best thing to do (if you have enough avios spare) would be keep your current booking and book the single F seat on avios separately. Then if a second opens up, you can also book that and then cancel the two other seats in J. If not, then as you say you can cancel the F seat up to 24hrs before.

But if you are on a 241 then there's not really anything you can do at this stage, just accept flying in J which is fine.