Transition from Synology DSM 6 Plex to Syno DSM 7 but with Plex under Docker

I cannot comment on the migration path from native to Docker as I do not run Plex via Docker.

Regarding DSM 6 & 7, they are very different beasts. There are many changes when running Plex as a native app on DSM 7 vs DSM 6. You cannot simply upgrade to DSM 7 and have everything work.

This is the definitive guide: