It is definately possible to do so (at least I know it is to go from non-nuke into the nuke field). I just did it. Due to some wierd circumstances I signed up as ACEF and then I reclassed into the Nuke Field. Some recruiters and classifiers don't want to do it because it is more paperwork, but you can definately do it. I warn you now; however, that you will spend ALL DAY at the MEPS station because the classifiers typically bump you to the end of the list so they might have a chance of not doing the paperwork. I was there from 0700 to 1900 hours.
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smoothtoaster