So with the formal announcement of new equipment to be built to replace the Acela's on the Northeast Corridor I am wondering what happens to the current equipment. Seems like the natural thing would be to keep the trainsets and assign them to the Northeast Regionals, thus allowing at least some of the more tired Amfleet cars to be retired.

A lot of railroads in the glory days used this model for passenger cars. The brand new trainsets would go on the crack train of the line, then when the next new equipment came on the scene, sometimes only a few years later, that equipment would go to the next secondary train that the railroad operated.