Surely it's the organisers perogative to decide whether numbers can be swapped? Some organisers have enough hassles on the day of an event without a bunch of runners turning up expecting to be able to swap details of the entry.
If an organiser (3 peaks springs to mind), wishes to accept numbers being changed then fine, if not, then respect the organisers decision, leave them to get on with the job of making sure the race goes off smoothly and find another race that you can enter!
</rant> :)

