Got V10 this morning and signed up for Spotify Premium. Then used Soundiiz to convert my Slacker Premium custom playlists to Spotify. You can also pull in song titles from other music sources via Soundiiz.
You don't need the paid version of Soundiiz as the free version will convert one playlist at a time. Took me about 30 minutes to convert my entire Slacker collection. About 3% of my song titles would not convert due to the titles not matching exactly, but it was easy to search Spotify and manually add them back in. Also, about 8% of my songs had newer remastered versions on Spotify, but the original versions got added. It's a matter of searching song titles manually to see if a newer remastered version is available.
Finally, because Soundiiz requires your account password for both Slacker and Spotify, I deleted my Slacker account and renamed the password on my Spotify account. Also, re-check your Tesla sound equalizer and balance adjustments as mine changed with the V10 update.