Users will be informed with enough time ahead of the Google Play Music service being switched off. Until then, both services will run in parallel. To make the switch to YouTube Music as easy as possible, Google has built in a "transfer" feature that automatically transfers all personal content from Google Play Music – including your uploads, purchases, saved songs and albums, personal and subscribed playlists, likes and dislikes, curated stations as well as personal taste preferences – to YouTube Music.
Source: https://youtube.googleblog.com/