The primary purpose of the rebuild, aside from overhauling 20-30 year old equipment, is to eliminate the air intake conflict with locomotive exhaust which prohibited them from being coupled directly to the engine in passenger sets. The 900s don't need that update because they were built to that spec. It is certainly reasonable to expect the 900s will be rebuilt when their time comes but there's no way anyone can possibly guarantee it nor who the vendor will be. There is also no compatibility issue with any of the power door enabled equipment and everything on the property is equipped with power doors despite much of the fleet being deactivated.
Moderator: Massachusetts Bay Transportation Authority, Brightline Trains
Avatar:3679A (since wrecked)/3623B (now in service as 3636B).