title: How can I change only the pixel format with ffmpeg?
body: <p>I have a <code>prores</code> encoded file with 422 pixel format and I'd like to convert it to 420 subsampling or such, to see how the file size changes. But seemingly, no pixel format except the default one is compatible. Here is the media info:</p>
  
  <pre><code>Duration: 00:00:20.02, start: 0.000000, bitrate: 253084 kb/s
      Stream #0:0(eng): Video: prores (apch / 0x68637061), yuv422p10le(bt709), 2048x1152, 200717 kb/s, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 23976 tbn, 23976 tbc (default)
      Metadata:
        creation_time   : 2015-10-13 14:41:12
        handler_name    : Apple Alias Data Handler
        encoder         : Apple ProRes 422 (HQ)
      Stream #0:1(eng): Data: none (tmcd / 0x64636D74), 0 kb/s (default)
      Metadata:
        creation_time   : 2015-10-13 14:41:12
        handler_name    : Apple Alias Data Handler
        reel_name       : A003R0D0
        timecode        : 167:47:51:21
  </code></pre>
  
  <p>I tried </p>
  
  <pre><code>ffmpeg -i raw_sample_0.mov -codec:v prores -pix_fmt (yuv420p10le | yuv420p | many others..) ~/Desktop/foo.mov
  </code></pre>
  
  <p>but none of those work. Is this because the codec simply does not support any other format? If so, how can I check which formats are supported and how can I losslessly convert to another format for which I can change the color sampling?</p>