OK, here is a direct link to the new custom version which I've called ExportImportAltKey,
See Open a different iTunes Library file or create a new one - Apple Support and open your old library that has the right metadata.
Select the files with the correct metadata and run the script which will export a file with the details.
Revert to the current library database.
Select the same set of files in the current library.
Drag the text file created earlier and drop it onto the script.
This will start importing properties from the text file to the current set of files.
The script is set up in such a way that you can see each proposed property change and decide to allow it, abandon the run, or allow each further change to happen without prompting. It should be clear after you've looked at a few if it is doing the right job or not.
You should really have a full backup of the database and media files before you get started.
tt2