Coaches on Downeaster will be the same as the NE Regional, it uses Amfleets from the same pool of equipment. On the DE, they fix half the seats in each direction, since the trainsets change ends without being turned.
Only difference is the Business Class on the Downeaster is in the cafe car (2-1 seating in half the cafe car).
If connecting via public transit in Boston between the Regional and North Station for the Downeaster, exit the Regional at Back Bay and take the Orange Line (towards Oak Grove) for a one-seat ride to North Station.