I have only purchased once but I think I recall completing the transaction, being told what my seat was, and then having the change-seat option, same as a regular booking. When I perused the available seats to change, there was almost nothing open.
Depending on the length of the flight, I think I'd be much more beside myself if someone was able to upgrade me and didn't just because Y was "together" and J wasn't. I mean, the seats in J are not really all that together anyway.