Seat changes typically happen only rarely, with any airline. As a LH HON, I have tried to have seats changed in my favor so my wife and I could sit together, but reservations agents always outright refused to do anything about it (which I found absolutely ok). The few cases where it happened to me (against my will) could typically be identified as computer glitches.

Interestingly it happened to me last on an Etihad flight in paid first class. My travel agent had put me on a middle seat (in a 1-2-1 configuration). At check in, I found myself in a window seat (I assume they needed the middle to accomodate a couple travelling together). It was done without EY asking me, which i found very astonishing in first class.