As long as you have your full metadata in the “Plex” named shared folder
Uninstall w/ Erase (cleans out the DSM 6 → DSM 7 upgrade mistakes
When everything is cleaned out, the next installation will automatically start Migrating (It will print errors or sit at “installing” for an extended period.
In conjunction with the instructions printed by the installer in its messages (which are tailored to your system) the above llinked instructions will guide you