No. You do not have to upgrade.
A Plex Pass enables additional features, no matter which version of Plex Media Server is installed.
It also lets you download beta releases of Plex Media Server.
The details of new features / fixes in each PMS release are available in the release notes
.
No. Those versions were always listed.
The DS923+ has an AMD CPU (tech specs). Use the build for AMD CPUs.
