They finally announced how this is going to work today ... basically you can sell unwanted (as if!) tickets back to them, buy returns from them and, subject to certain conditions, transfer tickets to another named person.
However, there is no formal 'swap' facility ... and they've stopped calling it an 'exchange'.
Full details are on the page below, at the foot of which there is also a link to ourworldcup.com. If you agree that 8% of the tickets available for fans of each of the 2 teams at each match is too low, please take a moment to add your name to their petition.
Here's the FIFA announcement:
The World Cup in Germany - FIFA ticket portal
