You're not alone. I have been flying since the 80's and a flight my wife and I are booked on in a month has seen 3 seat changes-from aisle/aisle to middle/window and then middle/midle in separate rows. The last change even was a change in flights from morning to afternoon.

After calls each time I 'pushed' UA to put us back where we were. The final flight where the flight change took place actually took us from a mainline to an E170, while the original flight went from an E-170 to mainline 319. I was able to get us back to the original flight and at least aisle/aisle behind. They had no explanation so I check very regularly now.