FYI, the response from Swiss was that this was due to an aircraft change (it wasn't, because it was operated by the same A220-300 as when I reserved my seat a few days earlier) and that they were able to move me to a similar seat, so no harm done, thank you very much and goodbye.

I replied that there was no aircraft change and that a seat 17 rows behind is not that similar, so they should at least refund my seat fee.

They then sent me a link for a refund of the ~$20 fee, where I was actually offered a $50 flight credit valid for three years, so I took that.