I had this problem also; needed to revert to iTunes 12.3.3 due to removal of convert ID3 Tags feature. I found that the following steps allowed me to do this; it only works because I had a Time Machine backup with 12.3.3 on it (happened to be the day before the 12.4 version appeared on the App Store). It is also highly recommended that you do another Time Machine backup before performing these steps so that you can recover back to the unwanted 12.4 if you need to again.
(1) Stop the itunes helper app using activity monitor utility if it is running
(2) delete the current itunes.app with terminal command:
sudo rm -r /Applications/iTunes.app
(3) Restore iTunes 12.3.3 from appropraite Time Machine backup.
(4) ALSO restore, from the same TM backup, the following files in the ~/Music/iTunes folder:
iTunes Library Extras.itdb
iTunes Library Genius.itdb
iTunes Library.itl
iTunes Music Library.xml
NOTE: not sure if all of those are necessary to restore but wanted to make the folder as close as possible to its pre-12.4; definitely the .itl is necessary. If you don't restore these you will get a message when you start the older iTunes that it cannot read the music library because it was created with a newer version of iTunes.
After performing the above, I was able to run iTunes 12.3.3 with music library still intact and it worked fine, including the Convert ID3 Tags feature.
Keep in mind, though:
(1) You will get prompts to re-update to iTunes 12.4; I have auto update disabled, so I just ignore the update in the app store updates list.
(2) The next time your version of mac os x is updated, it may contain a newer version of iTunes, so you would have to repeat the above process, only using a newer TM backup of 12.3.3 in case the content of the .itl and .itdb files has changed.