I tried this and iCloud Music Library made a mess of my music. All my FLAC files were replaced with AAC files. I expected as much, though I have a lot of music that has unusual meta data. I have a rare early EP album from Björk. The US release has different track order. iCloud Music separated the EP into 2 albums and promptly named the second album artist as Bj@?’k or something strange. It took a whole day of mucking around to get iCloud Music to stop messing with the import. I finally had to go into The Get Info for the album and select that this album is a compilation, which stopped this. Meta data and album art are the two biggest pains. Some of my U2 albums now have strange album artwork of music by Chris Botti. I do like Chris Botti, though how Apple decides it’s selections these days is perplexing to me.
I tried a year or so ago to move my iTunes/Music App Library to a separate partition on NAS storage to free up space on my old MacBook. Don’t get me started on that nightmare.