And, to address the specific question asked: no refund, no credit -- the fee is forfeit if the seat is changed but the ticket isn't.

I can't point to any specific bit of documentation on this, but preferred seating charges fall under a completely different payment scheme than tickets, where OP points out that airlines are often letting people have credit for things that aren't refundable.

This is intentional, BTW, not an oversight by the airline -- they know that if they allow people to request credit for a preferred seating fee, people will pay the fee to reserve the seat and then cancel just prior to check-in.