This is still basically starting over from scratch. The XML file doesn't just keep track of playlists, it's the DNA of your iTunes library, so where every single file is on your external. Even if you had to go back through and delete the older music, it would still be faster than indexing the whole library again. If that's not your forte, then you can choose to import the music again, only make sure the "Copy Files to iTunes Media Folder" box is unchecked. This will build the index again, this time with all the files you currently have; only, if you have quite a bit of music the process will probably take several hours.
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Also, this entire process is easier if you think of iTunes as a single body, not just an app that reads your music files from a folder. At its very core, that's what it is, but it's also an organizing powerhouse. So, if you want to declutter your music stash, it's much easier to do it from iTunes itself instead of trying to do it in Finder (or Explorer if you were on Windows). Every change you make, iTunes keeps track of and maintains folder hierarchy accordingly, which is much easier than manually doing it yourself. I hope this helps.