I just want to crop 2 pixels (Top) to get identical black bars (now is 138 TOP and 140 BOTTOM) but without re-encoding. I think it is possible with FFmpeg but I'm not sure. I want to do this with one H264 video (m2ts from blu-ray).
You can use the h264_metadata bitstream filter.
ffmpeg -i in -c copy -bsf:v h264_metadata=crop_top=140 out