[FFmpeg-user] Is there a way to re-encode an inefficiently encoded file without losing quality and reducing file size?

[Stéphane Archer]

Let's say you have a video file encoded with high bitrate with the
ultra-fast preset in H.264 because you couldn't afford CPU at that time.
Now you have a huge file mostly because it was encoded quickly.

You are now two days later with access to a good CPU and GPU.

If you re-encode that file to save space then you are going to lose quality
because even AV1 with a high bitrate is lossy (and you want to reduce that
high bitrate)

Otherwise, you can try transcoding using lossless compression but during my
testing, this is making the file way bigger no matter what I tried.

So basically, I can lose quality, or keep the big original file...
Do you know a better way? Is there a way to re-encode that inefficiently
encoded file without losing quality and reducing file size?

Best regards

Stéphane Archer