Seabourn can and should deal with this on your behalf with DL. If there are seats on the the flight you want in the right cabin, even if a higher fare class, Seabourn should be able to get this done for you since it was an involuntary change. However, if you have already made changes directly with Delta, that would limit their ability to work on your behalf. If that hasn't happened, get in touch with Seabourn, tell them what you want, and let them deal with it.