Change of plane and class - PE to Economy

Hi everyone. My boyfriend booked MAN - JFK through Delta in August to fly next March, and he's just received an email saying that our seats have been changed from PE to Economy both ways! Given that our outbound flights fall on my 40th birthday, my user name has never been more apt! It looks like it was due to a change of aircraft, but does anyone else have any experience of this happening? Will there be any chance of us keeping our original class?