user12267: My situation:

Transcoding 1080p bluray videos to HEVC. Audio untouched;
My rig is old (pre-2012) and to speed up the process I'm using --preset ultrafast;
From x265 documentation, I understand that video quality is independent of the preset used. And the slower the preset, the better the compression ratio it seems;
I'm using FFmpeg.

My Reasoning:

I want to start converting to HEVC now because the codec is good enough for me and the space saving is phenomenal;
I've seen people reporting a 35GB video compressed to 1GB or so with crf=18 and --preset veryslow. It sounds ludicrous but an experiment of my own shows about 40% additional space saving between presets ultrafast and veryslow.

The Question:
When I get a new rig with a more capable CPU (Skylake or later, I hear), is it possible for me, to "re-compress" my HEVC videos using eg. --preset veryslow, without quality loss due to encoding a second time?

user1871: In strict mathematical terms, no.
In terms of maintaining an acceptable image quality, yes. If you use CRF 18 and a preset like ultrafast now, you should be able to get a smaller acceptable file with preset veryslow later on.

12267: <p>My situation:</p>

<ol>
<li>Transcoding 1080p bluray videos to HEVC. Audio untouched;</li>
<li>My rig is old (pre-2012) and to speed up the process I'm using <code>--preset ultrafast</code>;</li>
<li>From x265 documentation, I understand that video quality is independent of the preset used. And the slower the preset, the better the compression ratio it seems;</li>
<li>I'm using FFmpeg.</li>
</ol>

<p>My Reasoning:</p>

<ol>
<li>I want to start converting to HEVC now because the codec is good enough for me and the space saving is phenomenal;</li>
<li>I've seen people reporting a 35GB video compressed to 1GB or so with <code>crf=18</code> and <code>--preset veryslow</code>. It sounds ludicrous but an experiment of my own shows about 40% additional space saving between presets <code>ultrafast</code> and <code>veryslow</code>.</li>
</ol>

<p>The Question:</p>

<p>When I get a new rig with a more capable CPU (Skylake or later, I hear), is it possible for me, to "re-compress" my HEVC videos using eg. <code>--preset veryslow</code>, without quality loss due to encoding a second time?</p>
 1871: <p>In strict mathematical terms, no.</p>

<p>In terms of maintaining an acceptable image quality, yes. If you use CRF <code>18</code> and a preset like <code>ultrafast</code> now, you should be able to get a smaller acceptable file with preset <code>veryslow</code> later on.</p>
