This issue makes me question the value of checking-in 24 hours prior to departure. For example, if I had checked in at the airport, seat 10A would have still been reserved for me, but I would have (almost certainly) known that the plane had changed; I could have then swapped to a seat in row 11 (it was not full on boarding), or selected some other alternative. If you have preselected a seat I now see little advantage to the passenger in checking-in so early.
I tend not to check in online when travelling in Y. I choose seats as early as possible (at time of booking or T-7days when Bronze) and then keep checking both the cost of the POUG (if offered) and the seat map in MMB for an early release of bassinet positions/better seats. When OLCI opens, sometimes it's possible to select seats with out checking in. At the airport without having checked in, I feed the self-serve machine with the things that it asks for to check the seat map again if I haven't already snagged what I would consider a preferable seat. If I haven't taken up the POUG offer, then it's off to the bag drop to check the AUP cost.