You don't need to re-render anything, at least not in Blender. If you have video editing software such as Windows Movie Maker, you can open your video files in that program and encode them in a different format. This is substantially faster than doing another "render within Blender."

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It is better to do one thing well, than to do many things and excel at nothing.

Forum: Blender

Subject: what is the best way to edit mp4 movies created by blender??