Per RR.net and confirmed by Amtrak employees...
The Downeaster, after months of rumors and some blowback from NNEPRA, is formally swapping out Amfleet coaches for surplus Midwestern Horizon cars. The interior livery is very similar, but the Horizons have harsh commuter rail-style lighting, no automatic doors, and an established rep for door problems during cold weather. They'll start getting added to sets one by one, so there'll be weeks to a couple months of mixed Amfleet+Horizon consists before they go all-Horizon. Cars will be shipped in/out of Boston the typical route for Downeaster equipment swaps: via Albany and extra non-revenue cars attached to the Lake Shore Ltd.
I guess start pining for the Airo cars in about 5 years, because this is somewhat of a downgrade.