Consolidating itunes libraries into one
"i Tunes in the Cloud is not a backup service," it says, "and having a local copy (a copy downloaded to a computer) is the only way to back up your purchased media." https://support.apple.com/en-us/HT201625 Then choose the Account menu.
Plus, we'd rather know that we have a safe and a complete backup of our media that we can return to. You have years of collected media that it might be hard or at least extremely time-consuming to replace —not only do you have to find the CDs you ripped but now you also have to figure out how to rip them again when no current Mac has a CD or DVD drive.
Apple keeps everything, all of your media and all of the information about it, in the one i Tunes folder so as long as that's safely backed up, it can be safely restored.
If you don't have the room for these duplicates on your internal drive, the default location that i Tunes uses, there is something you can do about it. You don't just run out of storage space through this duplication and consolidation.
Consequently, before you backup your i Tunes library, make sure it is complete and then do what Apple calls consolidating it first.
Go to i Tunes Preferences, click on Advanced and see that the option Automatically delete watched movies and TV shows is not selected.