So first off let me start by saying I am a nuke ET about halfway through prototype in Charleston so I've seen the majority of the pipeline. As far as re-rating, your recruiter won't tell you this but your contract doesn't really take affect until your on the plane to boot camp so they won't like it but you can cancel your contract before going to boot and not be charged or anything. That being said I've never heard of anyone transferring to anything else after signing nuke because it's such a critical billet they don't want to let you go.
As far as training goes it's really what you make of it. A-School deffinetly sucks the most (especially for ET's) but as you go it gets better. Go into it expecting to do 12-14 hour days every single day and to study on the weekends, that way if you get out before colors one day your super excited
. The pipeline is like this A-School --> T-Track (hold between a and power school, usually not that bad you get out early a lot more)--> Power school (life gets much easier if you work hard at first, I would go in at 0530 and leave around 1600 every day, no weekends)---> Prototype (the first 7weeks is kind of a classroom setting, 12 hour days but it's not so bad. The next 17 weeks you work rotating shift work on the boat which sucks at first but it's a lot more fun then power or a school.)
So many people say being a nuke isn't worth it, I disagree. Your paid fairly well and it's very rewarding to work those hours, it sucks at the time but you'll look back and be glad you did it.