when my X900E was faulty a couple of years ago, not only did Sony replace the TV as new, they replaced it with the X900F which had subsequently been released.
Maybe it's the cause of the issue that decides whether you get a new or refurbished replacement then.I believe they get classed as certified replacements ,however mine was 100% new and as you say they do give you the latest model if available ie my AF9 failed and was replaced with an AG9
I know of another Sony LCD owner on this site that has had 4 replacements over the years due to thrips. And all I believe were refurbished sets.
I know Sony are the only manufacturer that will replace your set with this issue, so maybe they see it as they're doing you a favour by even given you a refurbished replacement .