Not certain but:-
Yes you get the renewal letter for the same seat
And yes you can move but only after the renewal period for other season ticket holders has expired because they don’t know if an existing ST holder is leaving it to the last moment.
Once that period has expired you can move but you have to pick your time to do it because unallocated seats go on general sale and if somebody has bought a ticket for that seat for an upcoming game the ‘ system” will block you from having it.
The staff in the shop are very good at arranging the move but it’s a bit clunky for them because it’s an “un- user friendly” system they have to use