You can change the container/format of your video file without encoding if and only if your target container/format supports the video and audio codec of your existing video. Otherwise, you will have to re-encode the video with a new video/audio codec supported by your new container/format.
In your case, WebM format does not support H.264 video codec so you have to re-encode your video to either VP8 or VP9 codec which is supported by WebM format.
Refer to
these links for more information in detail: