Changing playback speed by changing frame rate
So you want an output video with the same frames in the same order, but with just the timing changed.  No problem.

ffmpeg can't do it without transcoding, unfortunately.  So you'll need a program that can muck around with the timing info in the container.  mkvmerge can do it, and so can MP4Box.

I knew I'd seen an answer to this when I googled on it before:
https://askubuntu.com/questions/370692/how-to-change-the-framerate-of-a-video-without-reencoding

From there: the guy eventually found the right MP4Box command line (since he was working with MP4.)

MP4Box -add test_track1.h264:fps=30 -new dest.mp4

(where test_track1.h264 is the raw h.264 stream he demuxed from his source.mp4)

If a Matroska container works for you, there's also the option of:

mkvmerge -o out.mkv  --default-duration 0:30fps --fix-bitstream-timing-information 0 in.mp4