I can't think of a reason to switch needles. I recently finished the front part of my sweater. I used circular needles throughout the entire armhole, neck, and shoulder shaping.
I'm confused about why you think you need to switch... :?: Are you knitting the sweater in the round?? Or are you knitting it flat? I've never knit a sweater in the round, so I can't speak to it. But I can't see a reason to switch for flat... :?