Decode/decompress H.264 back into raw/original file format, then encode into H.265

I have some files encoded using the H.264 codec.

There is a loss of quality when I convert them from H.264 to H.265.

I imagine I should convert them back to raw/original file format, then encode them into H.265.

Is it possible to decompress/decode H.264 into the original format (perhaps using FFMpeg)?

Is it the best way to convert from H.264 to H.265 without quality loss?

Thank you again for your help,

Solution

H.264 is lossy; the quality is lost at encoding time. There is no way to reconstruct the original from encoded form. In contrast, decoding is lossless - it produces exactly all of the information present in H.264 file, no more, no less. If your video editing software is not horrible, your H.264->H.265 conversion is the highest quality you can theoretically achieve given the compression settings you provide (without finding your original uncompressed file); there is no benefit in a separate decoding step, as that is what your software needs to do anyway.

Imagine a bad photocopy: there is no unphotocopier that can give you the original. That's what is happening with lossy compression.