Assuming a 64-bit version of the server is available, can I just install this over my 32-bit version of the Plex server, or is it a new build from scratch? e.g. will I lose my configurations.
This is also on the premise that there is benefit such as performance, stability, or additional features in doing such an upgrade.
As far as I know, unless it happened quite recently, there is no Windows 64 bit version. Also they have said that there are no plans for one as they say, and I believe they are correct, the advantage is too tiny to justify the trouble. It is not Plex, mostly, that slows things down but rather it is disk i/o, network transfer speeds and, in remote access situations, internet speeds.
I just wish Plex would fix their app so it works correctly. That would do so much more to improve things that the marginal improvement of moving from 32 to 64 bit ever could.