Thanks for asking, ofcourse you can add them back. Just head over to spotify.com and after logging in using the family plan manager account you will find an option to see your premium family, add her back by sending her the invite and once she clicks on it and enters the address, there you go! She will be moved from free to premium plan with all her playlists, artists and liked songs as they were. Make sure to claim the family invite using the account you want to add.
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