May I suggest
If the metadata is still in the “Plex” share, then please
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Unininstall the package with the ERASE option (to remove the errant DSM 7 installation)
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Now install any version of PMS from Plex.tv/downloads
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Migration will automatically start.
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it will sit at “Installing” until done. Don’t restart the NAS until it’s done.
If DSM tells you "Operation Failed’, ignore it.
Open Resource Monitor → Processes (or Task Manager) and look for PlexMediaServer (the new name).
Also, look in the Plex shared folder for Migration.log which will chronicle the progress (takes about 2 minutes to show up after you start installing)