They remain in the playlist but show as unavailable.
If you’re simply migrating from Tidal <—> Qobuz, with a few widely available local tracks, then as Suedkiez suggests you can can use Soundiz and as long as the new service has the same tracks in its catalogue you’ll end up with a new Qobuz or Tidal playlist, which you can then save a local copy of in Roon if you want to edit it.
You’ll now potentially have three playlists in Roon: your original one, a new Qobuz/Tidal one you created via Soundiz and possibly another local copy you’ve created from it in Roon. At this point you probably want to delete the original one and maybe also the original Qobuz/Tidal one if you’ve already created a local copy of it.
Things get more complex if your playlist has local tracks that aren’t on Qobuz/Tidal or you want the new playlist to retain any pointers to local files/versions over the streaming platforms versions, as currently there’s no easy way of handling those two scenarios…