You are much more likely to go to Swanwick if you want to. Although I do know of a few cases where people have got Prestwick when Swanwick was there main choice. It led to at least one person to end up quitting as Scotland was not an option for them.
The vacancies will not be filled by people from MACC moving up though as the only thing that will be changing is the location people will be working. They will still be doing the same job. If anything as postings to Scottish and MACC have been stopped in preperation for the move, they may be taking on more people up there than normal in the second half of 2010 to make up for the drop in new trainees going there and the number of ATCOs leaving Manchester instead of moving.
While there is a much greater chance of going to Swanwick, there are no gurantees and you should be prepared for all possibilities when you begin your training for NATS.