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