Originally Posted by
ratechaser
Data point (FWIW) - I had a December seat booking locked in while silver, I was downgraded to bronze (incorrectly, but that's another story) and not only did it stick, but when there was an operational shift to those seats, I was then able to reselect new seats for no charge.
Somewhat similar data point
As Silver, I booked flight with my kids who have Blue status. I chose seats for all of us when booking the tickets. My wife and other child joined us later, all of us to return together. I had to change my flight so that they flew but I did not. I changed my flight 3 days before departure. Day 4 prior to, the boys and I had been upgraded on one of the legs to WT+. They retained their seats and the upgrade. FWIW.