I'm trying to change the framerate of an MP4 video (it's a 120fps GoPro video, I want to play it back at 30fps for a slow-motion effect).

I'm using avconv for this:

avconv -i SourceMovie.mp4 -vf setpts=4.0*PTS -r 30 DestMovie.mp4

That technically works, but it reencodes the movie. In addition to being slow, it's obviously a quality issue. Technically there should be a way to just set the fps in the header of video, how can I change that? (Any tool other than avconv would work too.)