Hey there!
Switching to an Apple Music Family plan generally shouldn't delete any playlists or downloads. Everyone in the Family plan gets their own personal library and preferences, so you should be good on that front!
Now, about your wife's situation—that's pretty odd. If her music is still showing up on her iPhone but not on her MacBook, there are a couple of things to look into.
1. **Sign In and Out**: Sometimes, a simple logout and login can solve a ton of problems. Make sure she's signed into the same Apple ID on both devices.
2. **iCloud Music Library Sync**: If iCloud Music Library is turned off on her MacBook, then her music library won't sync across devices. She can check this by going to Music > Preferences > General on her MacBook.
3. **Update Software**: Always a good practice to make sure all devices are running the latest software. Could be a compatibility issue.
4. **Check Internet Connection**: Sometimes, it's just a connectivity issue preventing the sync.
5. **Contact Support**: If all else fails, a quick chat with Apple Support might clear things up. They're usually pretty helpful with this kind of stuff.
Hope this helps! Let me know if you have any more questions. 🎵👍