This has happened to me before. An aisle seat selected at booking on a Saver fare was changed to a middle seat at check-in. That was all it took for me to bite at a 99$ Premium Class upgrade SFO-JFK, which went with the middle seat empty so a win-win for both me and AS.
In more or less the reverse situation, another back of the 'bus aisle seat selected at yet another booking on a Saver fare from SFO-JFK, got automatically switched to a Premium Class aisle the day before check in (I'm not an AS or partner elite). Sometimes AS giveth, and sometimes AS taketh away!