I don't doubt that you can still enter an FAS. RBA programs have their own FAS's, so freshmen wanting to take an FAS who enter (or exit from) the Next RBA program following REX will obviously have to switch if they were previously assigned an FAS.
I'm a traditional advising alum, though, so I'm afraid I don't really know much else beyond that.
You should look at the advising booklet you were given to see if that can answer your question. You could also forward your question to
firstyear-www@mit.edu.