Hi @Driftm3,
Thanks for your reply! I would be glad to run through a scenario; these are great questions.
If you switch to the Premium Duo plan, you are right that everything will look the same as usual for you; your saved music, playlists, and recommendations shouldn’t be affected by your subscription plan. The only difference in terms of what you see on the app is Duo Mix, a playlist that Spotify creates containing genres, artists, and songs that each member of the Premium Duo plan have played on their own, and it updates the more you play. So, Your Library in your new Spotify account will still display the playlists that you created and that your wife created on your account before switching to Premium Duo.
Since your wife will need to create a new Spotify account in order to be invited to the Premium Duo plan that you start, she won’t be able to find the playlists that you created or that she created from your account upon first logging in to the app. If your wife would like to save these playlists to her library instead of starting from scratch to recreate them, I would recommend following the steps in this Spotify Answer regarding sharing playlists from one account to another (you can ignore the last part, “Here’s how you can close your old account,” since the article was originally written for the purpose of Spotify users creating a new account for themselves to replace their current one).
After switching to Premium Duo, if your wife or you create a new playlist, it will only show up for the individual who created it on their own account. However, you can always follow the instructions in this Spotify Answer to share a new playlist created from one account with the other account in your Premium Duo plan.
I hope this helps, and let me know if you have any other questions.
Have a good day!