You can transfer playlists created in an old account to a new one:
- Log in to the web version of Yandex.Music using your old username.
- In the upper-right corner of the page, tap My music.
- On the Playlists tab, tap the name of the playlist.
- Under the playlist name, tap and select Public using the toggle button.
- Tap and copy the link to the playlist.
- Sign out and log in under your new username.
- Tap the link to the playlist.
- Tap and select the playlist that you want to add tracks to.