You are correct; /volume*/Plex no longer contains anything about Plex Media Server
Item 5 above.
The Plex share is now free for you to do as you wish.
Did all your existing server migrate?
If so, The only additional work for you to do now is, instead of granting user Plex permission to read your media, you grant permission for System Internal User “PlexMediaServer” to read that media.