As a long time UNIX sysadmin this still blows me away. They converted over a billion devices to a whole new filesystem successfully, and not only that, they did it with a point release, not even a major iOS update.
This gained me a lot of additional respect for the OS team at Apple. They absolutely know their stuff.
Yep. Part of how they did it is that a previous minor release included a dry run, where the OS would try the conversion, collect diagnostics, then undo it.
All users really noticed about that was an unusually slow upgrade.