How to change metadata with ffmpeg/avconv without creating a new file?

You can do this with FFmpeg like so:

ffmpeg -i input.avi -metadata key=value -codec copy output.avi

Example:

$ du -h test.mov 
 27M    test.mov
$ ffprobe -loglevel quiet -show_format out.mov | grep title    # nothing found
$ ffmpeg -loglevel quiet -i test.mov -codec copy -metadata title="My title" out.mov
$ du -h out.mov
 27M    out.mov
$ ffprobe -loglevel quiet -show_format out.mov | grep title
TAG:title=My title

See the documentation for -metadata and on stream copying for more information.

Note also that not all formats allow setting arbitrary metadata, for, e.g., Quicktime doing -metadata title="my title" does what you’d expect, but -metadata foo=bux does nothing.