I had a hell of a time with the 1067 Anniversary update. I tried over and over for months. I had added a SSD. I ended up having to unplug D:, an HDD.
Post Upgrade my D: was changed to F:.
Interesting. You're not the first person I've heard of who had drive letters change post-upgrade.
Woe to the poor user who installed had any applications on a drive that got changed -- the applications won't run any more from the new location (all sorts of Registry paths are wrong), and you can't even un-install the applications because they're not where they used to be, so the uninstalls fail.
It's not quite as bad with data files, but it's still a PITA.
I have no idea why this happened to you, or those folks, but it seems real odd.