As OXONEIL says, I would doubt everyone would have to start from fresh. Also this would contradict an email sent last Aug from FGW advising they intend to have positions available early 2015. (Anytime between now and June)
I was told in my DM that they were looking for 300 drivers across the network. Especially at depots like Reading that were expanded. In addition to this my understanding and was told, is the new contracts are TUPED to Crossrail When it opens. Makes sense. Due to open soon, gives time for drivers to train up, qualify and gain experience. Therefore come open day, there isn't a let down due to shortage of drivers.
My mate was lucky to get on the course that I just missed out. They said they have still not met the required amount of drivers in their recruitment drive and are half way. I think most of what you've said is probably right but starting from scratch is over the top. If they did start from scratch it would be a bit pointless given half of us in the holding bucket would re-apply again with our passes...
Also since my mate started one thing I've learnt...even though candidates get onto training courses, not everyone makes it. Members on here like TDK, Driver455, etc make good remarks about those trying to 'score points' in the assesment stages. Believe it or not, the training isn't what a lot of people think it is...from my mates course and courses before them, lots of failed ( severely ) or dropped out as they are struggling with both home life and the training. In addition to those in the hold pool. Some drop out. Offered elsewhere, different job, circumstances, etc
I wouldn't be surprised if they actually start a new trainee driver advert in addition.