I just looked at our flight to LIH this weekend. I had booked us into 17E and 17F originally (and double-checked the confirmation email to make sure I wasn't just imagining that). Presumably at some point between buying the tickets and now, they swapped the aircraft. Instead of putting us into 17B and C (which were available) they punted us back to row 31. No notice, no email, nothing. Lucky for me I just checked the seats and was able to move us to 17B,C instead of row 31.

I don't know what to do for the future though. Are there alerts somewhere that tell you your seat changed out from underneath you?