ffmpeg -i input.flac -map 0:0 -map 0:1 -c:a libopus -c:v copy out2.opus
Input #0, flac, from 'input.flac':
  Metadata:
    TITLE           : Want You
    ARTIST          : Moe Shop
    DATE            : 2015
    COMMENT         : Visit http://moeshop.bandcamp.com
  Duration: 00:02:14.68, start: 0.000000, bitrate: 1080 kb/s
    Stream #0:0: Audio: flac, 44100 Hz, stereo, s32 (24 bit)
    Stream #0:1: Video: mjpeg (Baseline), yuvj444p(pc, bt470bg/unknown/unknown), 700x700 [SAR 72:72 DAR 1:1], 90k tbr, 90k tbn, 90k tbc (attached pic)
    Metadata:
      comment         : Cover (front)
File 'out2.opus' already exists. Overwrite ? [y/N] y
Stream mapping:
  Stream #0:0 -> #0:0 (flac (native) -> opus (libopus))
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
[libopus @ 0x55b8a94c4c00] No bit rate set. Defaulting to 96000 bps.
[opus @ 0x55b8a94aa380] Unsupported codec id in stream 1
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:0 --
Conversion failed!