Every time a new Rotem control car enters service, a 1700 goes north and a 1600 gets sidelined. AFAIK none of them are retired at this time, they're stored serviceable.
I suspect the decision of which goes next is relative to their blue cards. Control cars are engines in the eyes of the FRA, which is why they're more expensive to run even if not as a controlling unit. It's the reason 1600-1624 were deactivated years ago, and why most of those are still in service.
Moderator: Massachusetts Bay Transportation Authority, Brightline Trains
Avatar:3679A (since wrecked)/3623B (now in service as 3636B).