This happened to me on my JFK-NCE leg in DeltaOne. Had booked 3B & C back during the Skymiles flash sale in December. Now I've been pushed back to 9B & C. The only option I have is 1B & C or change flights. I have a connecting flight NCE-ORY so that's not a good option and probably not many seats available for a late April departure.
I don't know what the acronym FAM stands for but I can certainly guess. My suspicion is that someone at Delta with the appropriate level of access to the res system put someone who is a (pick one or more: paying, famous, higher status, big complainer) and the system had to resolve the duplicate seat assignment by moving me to the back of first. At least they kept my wife and I together.
Has anyone on FT had any success getting their original seat(s) back? Or getting some SkyShekels as compensation? Yeah, I know this is a first-world problem but I think DL ought to do something for me having to read automated emails and look at apps (which still show my original seat assignment) and make a call to find out what's going on.