I'd be very surprised if you could hold them to it as service past 12 years involves being 'boarded' and all that in-tails (ability, OJARS etc). It would be wrong to have a 'pass' when others were effectively in competition with each other.
On a more pos note, your exit date is after 2015. Whenever there's been a mass cull in the past, there has always been a huge shortage afterwards with the service desperate to retain people. Also the the next SDR is in 2015 so it could be all change by then.
My view is that in your position you are probably safe, they won't want to make you redundant as you still 'owe' them 6 years service and it won't cost them much when you leave at 12 years. Just don't expect much job security between now and then.