I think it’s possible to play the game even at a main cabin fare, not selecting a seat and hoping for only comfort available later at check in
Now you are into different territory, and I doubt that would happen on any airline. What you would need to hope for is an oversold situation, but in that case, people with status, in comfort or premium will be upgraded to first, then people with status in main will fill comfort seats, and you get what seats are vacated. There is just a pecking order to class upgrades, and that happens in the last half .
hour before boarding, unless you are awarded an upgrade earlier due to status.
So yes, you can move from random seat to an exit row, front of the plane, or other seat usually available to main cabin and someone with a bit of status, but not going to change classes of tickets just by checking in late.