To reencode them to proper 720p, use this:
ffmpeg -y -i <infile> \ -acodec copy \ -vf "pad=width=1920:height=1080:x=4:y=4" \ -s hd720 \ -vcodec libx264 -preset slow -threads 0 \ -x264opts fast_pskip=0:crf=20 out.mkv
This pads the video with 4px wide bars on each side and resizes it to a proper 16:9 (1280×720) aspect ratio from 1080p.