share your steps man share please!
Backed up all songs on iTunes for Windows on a USB drive.
Deleted all songs and playlists from iPhone.
Rebooted Windows laptop, hard rebooted iPhone.
Used Lightning cable to do a virgin sync of iTunes Library to iPhone. Did a second backup + sync just to make sure.
Disconnected iPhone from Lightning cable, connected laptop to Ethernet cable for fastest speed possible.
Stayed in iTunes for Windows, chose Edit > Preferences > iCloud Music Library and checked the box.
Waited 2 hours for iTunes to compare my 15,000 songs to the Apple Music library and upload what it needed from my boots and rarities. When the progress bar was full and the sync was complete, I closed out of iTunes for Windows.
Launched Music on iPhone, went into settings and turned Apple Music sync to 'on'.
From that point forward, both physical libraries on my Windows notebook and my iPhone are completely in sync. If I make a change on my iPhone, it changes on iTunes for Windows and the other way around too, add a playlist, delete a song, add a song, replace a song.
FYI, on my Apple TV, same thing. My ATV doesn't have internal storage so unlike the laptop and the iPhone its pulling straight from the cloud.
In this manner, I found that none of my songs were replaced by those in Apple Music at all. Now, that's good when it comes to boots and such, I didn't want those replaced. But there are thousands of old rips of common songs at 128kbps that I wanted to get replaced with brand new 256kbps versions and that didn't happen for many of them.
Hope this helps. Note this was my first sync ever to Apple Music. I never sync'd it before and never did iTunes Match back in the day. So my library was virgin in that regard.