1. Backup your library
2. Create for testing purposes a new library with Library Manager by cloning the old one and deleting its content.
3. Select all tracks and export to an *.mpl file in your home directory.
4. Edit the *.mpl file. Depending on the severity of your changes, you might be able to do it with notepad. If you need complex editing and have to make use of regular expressions, you need a more sophisticated tool. I use SearchReplace from
www.funduc.com. You can use a trial version, but it has a steep learning curve. You might try to use WinWord and save as a text file (never done it). Be sure that the file format is UTF-8.
5. Once the editing done, import the *.mpl file by right clicking on the file, select Media Center>Import. In this way you do not have to change your definitions in Import Media Menu. Just be sure that the *.mpl file association is set.
Notes: If you make changes to field names, you must recreate that field in the Media library first (Options>Media Library), otherwise it won't be imported.
Good Luck!