ffmpeg re-encode libx264 without quailty loss?
You have a misunderstanding of how compression works.  In all but a few specialized types of lossy compression, when you compress something a second time, even in a much higher quality level than previous encodings, you still lose additional quality.  

Using a slower encoding from the same original source with constant quality will often produce a smaller file of similar quality, but if you encode the already compressed version over again, you will still always end up with a lower quality video due to second generation quality losses from compressing twice.