You cannot do what you want (change the location of the media files) without directly editing the database. [HowTo] Plex database modification - Moving media the right/wrong way - #10 by jelwell
There’s another strategy, but it requires you to keep the number of drives the same.
In short:
- disable automatic library updates as above
- stop Plex server
- copy files from old to new drive, keeping folder names and file names exactly the same
- remove old drive
- re-assign the old-drive-letter to the new drive
- [repeat for the second old drive]