I’m running out of space on my server and I would like to replace as many episodes as I can with x265 encodes since the quality is the same or better than x264, and the size is a lot smaller. What is the best way to search for x265 episodes and replace my current ones?
The problem your going to face is equal quality, Sonarr won’t upgrade HDTV-720p x264 to HDTV-720p x265 and Sonarr would only upgrade the quality to a higher quality if the cutoff hasn’t been met already.
Setup a default restriction to only require x265 (applies to all series) to get only x265 releases going forward, backfilling old releases is going to require manipulating the existing files to reduce the quality Sonarr treats them and searching or finding better quality releases, which may use up more disk space.