I had a situation, posted in another thread somewhere on this forum, where I had an A fare for an ATL/SFO itinerary. I had booked this months in advance and it was the fare Delta offered me on their website when I asked for FC.

Just before they boarded the flight they determined the jump seat wasn't operational and I was the "lucky" pax who was sent to Y so they could have a seat for the FAs (an FAA requirement, I believe). The seat they took should have been taken from a last minute upgrader--at least in my opinion.

I called DL on my arrival in SFO and, after talking with SMS, was transferred to some sort of dispute specialist. The bottom line, they agreed that the thing wasn't handled properly and basically reimbursed me for 50% of the entire cost of the ticket. In other words, they essentially flew me out there for free. This was done partly as a direct cash reimbursement (about half of what they gave me back) and the other as "delta dollars." Although I don't know what would have happened if I had just written a letter, I am doubtful I would have been as happy with the outcome. After seeing what the OP got to his email, I feel more certain of that, although we will see what he gets with his letter.

I suggested earlier in this thread that the OP call and several of you responded that a letter would get better results. I disagreed then and I still, respectively, disagree.
I may have been "lucky" but the differing results, at this point, speak for themselves.