Thanks for your reply.
In NJ, probably it will be Newark. They are fast and efficient comparatively (2-5 months). This is what I could know from other postings on this forum - I don't have personal experience though.
Sometimes, they transfer without any reason ("Quality control"). Sometimes, past immigration status is not clear or current employment verification is required or medical report is fuzzy (say, positive for TB or HIV, then they transfer to local centers for proofs of treatment). Usually, they write in interview call (if any) what documents will be required during the interview. Sometimes (rarely though), local centers don't need personal interview, and send approval straightaway.
If you never changed employer, I guess you should not worry (except for additional delay of couple of months).
Best wishes,
Jigesh