My continued understanding is that the addition of the through sleeper has been postponed due to issues involving the servicing of a single Viewliner in Boston.
My guess is that they may not be calling passengers with existing reservations at all if they know they haven't worked this issue out yet. If anyone has gotten a call about changing reservations onto a through sleeper, I'd be interested in hearing about that.
I do wonder how much stocking parts is really an issue. At most hours of the day, a part that would fit on a Regional train could probably find its way from Sunnyside Yard to South Station in under 6 hours, if you count both the travel time (around 3.5 hours), and the waiting for the next train (maybe two hours in the typical worst case in the middle of the day?), though I guess this would be easier if there were more Northeast Regional trains with baggage cars. Then again, if they desparately needed to ship a part on a rare occasion, they might be able to attach a baggage car to a train that ordinarily wouldn't have a baggage car...