[Someone]

"Just make sure that your new directory is set up so that the same request for the same target file ends up requesting the same inode"

If that is your goal, why do the mv at all?

...until your program needs to work with more than one file and the two are related in some way or opens 'the same file' twice.

For example, your compiler could fetch file X from the 'old' directory and file Y from the 'new' one, or your web server could log that it fetched file Z (which does not exist in the 'new' directory) to a log file in the 'new' directory.

It may be possible to mitigate this by requiring everybody to access all fils true a directory you opened atomically for them, but even if it is: good luck enforcing that rule, especially when using third party libraries.