So to change seat you renew, then say you want to change at a later date and get put on a list?
That's exactly it.
Just renew yours now or whenever, then call up the box office as soon as you've renewed and let them know you want to change seats.
There's a week between the deadline for STHs to renew and general sale, so they'll call you in that week and give you first dibs on all the available seats that are left over from STHs not renewing.
When I moved I actually called up on the first day after the deadline and the girl was sound, she done it there and then, not sure how far down the pecking order I would have been if I'd have left it until they called me.