The only safe way to make the transition would be to edit the Plex database directly, and change all file paths to the other drive letter. But only for music items.
You will have to dive deep into the structure of the database to pull that off without side effects or data loss.
Make sure to always keep a backup copy of the database.
Shut down Plex server before making the changes.
Copy the music files to the new drive before editing the database.
See [HowTo] Plex database modification - Moving media the right/wrong way
