It took Google a long time to polish up YouTube Music, and there are still many die-hard Google Play Music users that aren't content with the company's new streaming product. But thanks to recent additions like file upload support and the Explore tab, Google deems YouTube Music to be almost ready to replace Play Music once and for all later this year. That's why Google has just started rolling out a migration tool allowing Play Music users to move over all of their content to the new platform, though they'll be able to access it on both services for the time being. There's also an additional workflow to move podcasts over to Google Podcasts.
Left & Middle: Switch banners. Right: Transfer workflow.
Over the next few weeks, people who use or have used Google's original streaming service will see banners rolling out on YouTube Music and Play Music, informing them that the old platform is going away. While that will still take a while, it'll soon be possible to import all of your GPM content to YTM already. Once the tool has rolled out to you (it's location-based), you can do that by visiting music.youtube.com/transfer, via the banners inserted into both apps, or by searching for a "Transfer from Google Play Music" entry in YTM's settings. Either way, you'll need to use the YouTube Music app or website to get the transfer going.
Once you start the process, a screen will inform you that all of your data, like music recommendations, playlists and stations, liked and disliked songs, uploads and purchases, albums and songs in your library, and your current membership and pricing will be copied over. Your recommendations will be imported first, so you can start browsing the YouTube Music homescreen populated with your personalizations right away. A banner informs you about the progress — when you tap it, you can see which content has already been transferred. You'll receive an in-app notification and an email once the process is completed, which can take up to a few days depending on how many songs you've uploaded. Your original library remains available on Play Music, and it's possible to repeat the migration at a later time in case you continue using Play Music after the first import.
You can start exploring YTM while the migration is in progress.
Google wanted to keep the transfer tool as simple as possible, so it's an all-or-nothing affair: You can either copy all of your content or none of it. You'll have to delete things you don't want to find in YTM after the fact. While all songs you paid for up-front will be moved over, you might lose access to some subscription content that isn't available in YouTube Music's library, which I can confirm for some songs on one of my playlists when I migrated.
If you're one of the people grandfathered in on the $8 plan, you'll be able to stick with it. Pricing also won't change for most people using the tool, but some countries with fluctuating currencies will see a price change. The migration process is also available to those who use the free tier of Play Music, and they can continue using the service without paying a dime. Their own uploaded and purchased songs are available for offline and background playback, too.
Should you be brand-new to YouTube Music, you'll notice that some stuff is organized differently. Your uploaded music is walled off from the streaming service content and sits in its own tab when you search or when you look at your library. Stations aren't a thing in YouTube Music, so all of your saved stations will be turned into playlists. The songs in them will remain the same, though, so nothing's lost.
Content you play on YTM will also show up in your YouTube watch history, and playlists with music videos will show up on both platforms. (Update: Many commenters report that the shared history is no longer an issue, though for me personally, music I listen to still shows up in my YouTube history. YMMV, I guess.)
The migration tool is rolling out as part of a server-side update, and should become accessible for most people on version 3.65 of YouTube Music. It's available on the Play Store already, but if you still haven't received it, you can grab it from APK Mirror.