He said it will be a vacancy/displacement bid. 30 F/O vacancies will open up in IAD, and 30 F/O seats will disappear from ATL (therefore "displacements"). Same for captains. It should be in seniority order, so everyone will put their bids in, and the computer will sort it all out. What this means to me, though, (and I could have this all wrong) is that there is the definite possibility that F/Os who are senior to the current most junior CR2 captain could take the captain vacancy in IAD, and leave the bottom captain being displaced to the right seat; meanwhile conceivably a more junior CR7 captain would be untouched.
It would create a bit of a mess since this is different than a new base due to expansion. I feel for any more captain downgrades that might occur, due to an FO deciding that now is the time to upgrade. Someone please correct me if I've got this wrong.