I upgraded my DS918+ to a DS923+ and my previous Plex installation seems to run out of the box, is it pure luck or do we might need a new (DS923+ specific) version in the future? My past experiences with Synology NAS upgrades and Plex is well, mixed, so.. 
I didn’t know the x23’s were available.
I’ll review and update the Compatibility Guide appropriately.
For the most part, you should be fine.
DSM 6 - DSM 7 was the painful upgrade I’m sure hence “mixed” feelings. 
For this, just perform a DSM migration (reinstall and let it do the work DSM 7 → DSM 7) and as far as Plex is concerned there will be no changes required.
