I've setup MCEBuddy to convert all recordings to H.265 to reduce the file size of the recordings. I was under the impression that as long as the filename didn't change, Channels DVR would still know what it is, even if the file type and extension changed. Is there something else I should be doing to get this to work?
Extension needs to be the same too.
Is there a way to have Channels record as an mp4 instead of mpeg?
Just for reference: H.265 offers very little size reduction for resolutions up to 1080p over H.264. The real size differences are evident for 4K and beyond. As always, caveat emptor—or is that caveat encryptor. 
(From personal experience, when encoding for comparable quality, I have even seen H.265 files end up larger than H.264 encoded versions, from the same source material.)
You can move the mp4's to a separate folder delete the original entry and Import them.
I'm getting some file sizes 2-3GB per hour of recording. I'd like to be in the <1GB per hour neighborhood. Is there anything I can do to bring the file size closer to what I'm wanting? I'm okay with 720p if that would make a difference in what would be suggested.
Try adjusting your bitrate, and the quality setting. Also, you'll probably find that x264 and x265 are better than using your graphic card's hardware acceleration. Also, for H.265, you may want to look at Intel's SVT-HEVC encoder.
Are there settings in Channels somewhere to make these adjustments or do I need to use MCEBuddy to do the conversion?
Mcebuddy