In the opening post of this thread, you will find all the info , including the DSM 6 → 7 migration procedure.
To answer your other questions:
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DSM 7 is final release and so is the packaging. These threads have been open and pinned to the NAS & Devices sub-forum since that time.
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It is safe to update. You MUST follow the procedures. I can give you a head’s up.
a. Uninstall Plex for DSM 6
b. Upgrade to DSM 7
c. Reboot DSM 7 again after the upgrade. (there is a quirk in DSM 7)
d. https://plex.tv/downloads
e. Select theSynology (dsm7)packages in the selector. -
Follow every instruction carefully. ( The documentation I’ve written explains why this is important )
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Many have upgraded flawlessly on the first attempt. It isn’t really hard but IS detail rich.