That sucks, and I have no idea what happened. Could have also been an equipment swap that booted a bunch of people, and then a swap back. Hopefully DL can research and find out. I don't disagree that an agent somewhere screwed up and you got the short end of the stick, but DL isn't going to move you back to C+ seat unless one opens up somehow.

As a counter to the argument of "shouldn't the person booted from a C+ seat not be the DM" - there's no way for you to know who else is in the C+ seats (unless DL disclosed this to you). Maybe they are other DMs. Maybe they are people who paid for the seat. At the end of the day, every one of those people could make the argument that they're more deserving of keeping their seat vs. anyone else.

Personally, I'd have asked for more miles to compensate the switch, especially as a DM. Had you paid for C+ and been downgraded, they would owe you the fare difference or seat assignment cost, which IIRC tends to be more than $100 for a TATL. Hopefully your PNR has been noted that you were involuntarily booted from a C+ seat, and you can keep an eye on the seatmaps to see if a C+ seat opens up before the trip - then call in and get assigned to it.

Pie-in-the-sky you could ask for them to instead place you in J, or at least grant an additional GUC you can apply to the flight, but I wouldn't hold my breath for either of those.