morac

Migration worked for me, but had a number of issues afterwards.

1. I had to disconnect Movies Anyway and reconnect it to my primary Apple account. That worked fine, though movies didn't show up on any devices I had already logged into (just my iPad) until I logged back out and in again. I appears all MA movies migrated over. I have 404 movies in my MA account and 470 movies showed up in my Library, so that seems correct. I didn't check all of them, but there are ones I bought outside of iTunes showing up.
2. My Apple TV got into a weird state where it was both signed into media and not (I saw my Up Next, but wasn't logged in). I had signed out and it wanted me to sign back in using my iPhone, but doing so never completed. I tried rebooting my Apple TV, but that didn't help. I finally had to log in to media manually on my Apple TV. After that I logged out and was able to log back in automatically using my iPhone.
3. I had to open the Home app on my iPhone and iPad and tap the pop up saying to switch my home to my new music account a few times. That may be related to issue #2 above.
4. When I signed into Music on my Mac, I couldn't find any way to enable library syncing. Eventually it asked me if I wanted to and I said yes. I have a large number of songs that I previously matched or uploaded via iTunes Match back in the day. These initially showed up as "waiting" and then a large percentage went to "error", "ineligible" or "duplicate". I selected them all and chose "add to library" and most, but not all changed to the correct status (uploaded, matched, etc). Some are still showing as "illegible" that had previously been matched or uploaded. Apple does say it can take several hours for your Apple Music library to update, so I'll try again later. As far as I can tell all my playlists are there, including ones I added this week. I had to authorize my Mac again.
5. I got signed out of Game Center several times (see below). I'm not sure why exactly. When I checked I was logged out on my Mac, iPad and iPhone. I signed in using my secondary Apple account which worked, but when I check a little later I was logged out again. I signed in again. I'm not sure how long it actually stays logged in. It does show the correct Game Center profile in the App Store when logged in, despite the accounts being different, but I fear that it won't stay logged in.
6. When I went to update an installed app it made me enter my secondary account password on my iPhone and iPad (not my Mac). After doing that I needed to enter my primary account password to view my media account. I believe this is because it needs to log in as the account an app was downloaded under which was the secondary one. I’m hoping it remembers the password as and I don’t want to have to delete and reinstall all my apps to get them to update without requiring me to enter a password every time.

After updating my HomePod minis one of them got stuck “Not Responding” in the Home app despite working otherwise.  That persisted after a restart and even unplugging and plugging back in.  I eventually got it working by unplugging all my Home hubs including an Apple TV 4K and power off and on my iOS devices which I’ve never had to do.  This is not any better than 16.3.

The article says to pick a recovery phrase that’s easy to remember.  Apple generates the recovery code (not phrase) so you can’t pick your own.  It’s not something you can remember so you’ll need to write it down otherwise save it somewhere secure offline.  If someone gets the code and knows your Apple ID, they can reset your iCloud Apple ID password and effectively take over your account.