I've seen a few data points in the past with Delta refunding of the fare difference in addition to $200 for involuntary downgrades when you paid for a higher class of service so I don't think you are being unrealistic with your expectations and will likely be able to get even more than you are asking for.

As for why you were downgraded in the first place, I didn't look into your flight specifically but it's most likely due to an equipment change where your original plane had more FC seats and was switched out for a plane with fewer seats.