No. Soundiiz is doing a copy process (as much as possible with a matching system) from your source platform to the destination. So, your source playlists and favorites will not be deleted during the conversion/transfer process.
That means we are looking at your source playlists and favorites lists, reading metadata like the track's title and the artist's, and trying to find a similar element on the destination catalog.
We are not downloading/uploading/manipulating audio files; songs in your playlists or fav list must already exist in the destination catalog to be matched.
However, be aware that if you use any "delete" button on Soundiiz, that will also delete instantly on the music service, so be careful when deleting an element on Soundiiz.