backup-by-copying-when-mismatch is a variable defined in `files.el'.
Its value is

  • This variable's value is permanent if it is given a local binding.

Non-nil means create backups by copying if this preserves owner or group.
Renaming may still be used (subject to control of other variables)
when it would not result in changing the owner or group of the file;
that is, for files which are owned by you and whose group matches
the default for a new file created there by you.
This variable is relevant only if `backup-by-copying' is nil.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.