I've had similar experiences but there are two slightly-off point situations I would like to share:

1. This is not a seat change, but I was in a 737 between SFO-SEA on a full flight. Even all the overheads were stuffed. A man boards with a large carry-on but there is nowhere to store it--the flight attendants try to help him since he says he needs it as a carry-on to make his international connection. Did I mention the flight was late, too? So, after seeing their perplexed looks I offer to take my bag out of the overhead and check it so the other man can use the space. The flight attendants and the man thanked me very much. At the end of the flight, the purser asks me to see her. She gives me two 500 mile upgrades and arranges for 5000 miles to be placed in my account. I never expected anything, but this was great.

2. I am in a 737 SFO-SEA (again) in an exit row, aisle seat behind the two-seat row. A young couple boards at the last minute and they are seated in the middle seats of my row. I vaguely remember young love, so I offer my aisle seat in exchange for one of their middle seats. I figure this is my good deed for the day. But when I take the middle seat I find my foot area under the seat ahead is stuffed with carry-on. I ask the couple and they say it is not theirs. I learn it belongs to the young woman in the window seat--remember, she has no seat in the row in front of her. I ask her to remove the items but she ignores me since she is on the phone. I tell her she can either have the middle seat I am in or remove her carry-on from my area and place it under the seat two rows ahead of her. No response, so I dig it out and place it on floor ahead of her. That makes her hang up the cell phone and reprimand me. But, she grudgingly places her stuff under the seat way ahead of her. Maybe I was too quick in expecting her to get off the phone, but the plane was getting ready to go. So, she was mad the whole flight and took over the adjoining armrest. It was her problem. I stretched out and slept.