If an old configuration exists it is updated with any new default directories. Additionally, any configured directories that point to non-existing locations are reset by pointing then to the users home directory. This typically happens when the users removed the directory, so they likely don't want to use it anymore.
On the first run a user-dirs.locale file is created containing the locale that was used for the translation. This is used later by gui tools like xdg-user-dirs-gtk-update to detect if the locale was changed, letting you to migrate from the old names.
xdg-user-dirs-update is normally run automatically at the start of a user session to update the XDG user dirs according to the users locale.
The following options are understood:
NAME should be one of the following:
PATH must be an absolute path, e.g. $HOME/Some/Directory.
The XDG user dirs configuration is stored in the user-dirs.dir file in the location pointed to by the XDG_CONFIG_HOME environment variable.
The XDG_CONFIG_HOME environment variable determines where the user-dirs.dirs file is located.
xdg-user-dir(1), user-dirs.dirs(5), user-dirs.defaults(5), user-dirs.conf(5).