No compensation due, but you might as well write into customer care since the manner in how it was handled clearly bothers you.

I'd send something like "I understand seat assignments are not guaranteed, but how it was handled was less than ideal in my view. I'm not asking for compensation but would like my feedback to be passed along to management and the training department as an opportunity to improve service. Getting moved involuntarily at the last minute without any apparent reason feels like getting the rug pulled out by under me. I selected this seat several months ago at booking because it's my preference. I don't know if the agent moved me because they thought they were doing me a favor or needed to do so for some other reason, but in any case, proactive communication would have improved this situation. Asking me if they could move my seat, regardless if it was actually involuntary, is much better than merely handing me a new boarding pass with a new assignment."

Who know, maybe they'll toss a goodwill gesture or even pass it onto right department and actually incorporate that feedback.