Ok. Mission accomplished.
I'm going to summarize what I did for future reference, just in case (maybe somebody else will find this helpful).
1. Create a Smart Playlist with the following structure:
Be sure to add a blank space after each term, otherwise you'll be collecting albums that you don't want to include, as "Abbey Road".
Also you can note that I've added other terms in addition to "The" and "A". This is particularly useful if you have music in other languages.
2. Right-click on the Smart Playlist that you've just created and select 'Duplicate'. Repeat this step depending on how many fields you want to fix.
I suggest following a logical order to name each one of these Smart Playlists, just to keep some mental peace. So for example I created: 01 Fix albums, 02 Fix artists, etc.
3. Edit each of these copied Smart Playlist, modifying just the first column of fields. So, the first Smart Playlist will display Albums, the second one Artists and so on…
4. Download this script:
http://dougscripts.com/itunes/scripts/ss.php?sp=thistagthattag. Open the disk image, follow the instructions and install it.
5. Go to your first Smart Playlist, select all the tracks with Cmd+A, then go to the Script Menu on the Menu Bar, launch This Tag That Tag, select Copy on the first field, Album (or whatever other field you chose to fix in that particular Smart Playlist) on the second one, and Sort Album on the third one (or the Sort field related to your previous choice). Then just leave the "Delete this tag" field unchecked and click on Proceed.
6. After the Script finishes its work you'll be notified. Then repeat the previous step with the rest of Smart Playlists that you've recently created, just making sure to adjust the right option on the second and third field of This Tag That Tag, depending on the Smart Playlist that you are working with.
By the way, if you wonder why making different Smart Playlists for each Sort field that you want to correct, it's just a matter of time saving. Otherwise you will have just one huge Smart Playlist, and you'll have to use This Tag That Tag on ALL of its tracks as many times as fields you want to correct.
So at the end I just prefer to spend 2 additional minutes making individual Smart Playlists.
That's all you need to rearrange your music, following the old convention used by Apple to index the iTunes Library. Then you are free to delete the Smart Playlists that you've created. But if you want to avoid this "A" and "The" index problem in the future, don't delete those Smart Playlists yet and follow these extra steps:
7. Drag the content of all those Smart Playlists into a new, simple Playlist. Then select all of the tracks, press Cmd+I and go to the Comments field. Then write there something for future reference, as "Track modified with This Tag That Tag".
8. Now feel free to remove all those Playlists (Smart Playlists and the regular one that you've just created) and enjoy your new and clean order. If you want to collect all the new tracks that need some modification and that you'll be adding to your library in the future, then just create a new Smart Playlist like this:
Where "Tag" should be one of the keywords introduced by you in step 7. Also make sure that the first column includes all the fields that you want to take care of (personally I recommend selecting Album, Artists¡, Album Artist and Name), and that you have all the desired prefixes covered for each of these fields.
The End
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All of this might sound unnecessarily complicated and difficult, but believe me it's not. The whole process didn't take me more than 20 minutes, which is a good thing compared to how long it would take to manually edit every track on my library containing a field that starts with "The", "A", "Los", etc.
And finally, please excuse my poor grammar. Obviously I'm not native speaker.
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UPDATE: I've found that iTunes Match tends to restore your tracks' metadata to their original state (what a crappy cloud service, btw). So if you have it enabled, all this changes will be eventually undone. However, if you sync your iPod, iPad or iPhone with iTunes manually via USB –or just use the iTunes library–, it'll be ok.