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Right now, I have Spotify Free and my wife has Spotify Premium - how can we move to one Spotify Family plan?
You can upgrade to Premium for Family at https://spotify.com/family and then invite your wife by following the instructions from this support article: https://support.spotify.com/account_payment_help/premium_for_family/premium-for-family/.
All of your account details will remain the same, including any saved music, playlists, and followers. Each family member gets to keep their own individual account to play and save their own music. 🙂
Her subscription will automatically be switched over, so she won't be charged for Premium after joining the plan.