Considered doing this a few months ago. Depending on the size of your library, it’s a mixed bag. Some concerns:

• People on the Plex communities talk about the H265 conversion never being as good a quality as a conversion from a primary or secondary source. I’m not a quality purist, but it might be an issue for some pieces for you. Worth considering.

• I ran a test on a season of a TV show I had in 1080p, and the results were about a 70% decrease in file size for each episode, which was huge, but the problem for me was that it (an admittedly very old gaming desktop) took about 6 hours per episode to get that. Having a machine churn like that for the extended time required for a whole library introduces a whole host of issues re: hardware fatigue, heat, electricity, sound, tying up your machine during the whole process, and I’m sure plenty more I’ve forgotten to mention, that might limit the value of embarking on such a task.

• It’s generally considered to be much cheaper (and a better use of all of your resources) to just spend what you’d spend doing that on more storage. 16 TB drives running ~ $200 will last anyone except the most extreme 2% of DataHoarders very well for media storage for quite a while.

Anyways, I hope any of this helps. I ended up holding off. The storage space savings are tempting, but the path there is expensive (in more than just $) and probably not worth the investment for most.