Speaking of declining op-ups, I had an interesting experience recently, which may or may not be meaningful.

Girlfriend and I are on separate reservations in Y. We OLCI at the same time and pick seats next to each other. On the way to the airport I refresh my boarding pass and notice I've been upgraded to W. At bag drop GF and I check our bags in together and I ask if I could be moved back as I'd rather sit next to her. The agent makes a phone call and confirms yep, no problem, and prints my new boarding pass with my old seat assignment.

Boarding the plane and girlfriend gets the beeps and a new boarding pass for W. 'FFS! All that faffing for nothing' I think, until the gate agent takes my boarding pass and rips that up too, printing out a new boarding pass for the seat next to the GF in W.

The excellent check-in agent must have tied the reservations together to make sure we didn't get split up again, and then (I guess) they ran another upgrade process some time between bag drop and the gate and I was still marked SFU. Not sure if there's a moral to the story, but don't be surprised if you decline an upgrade only to be given it back very shortly after!