File > Import Movie > Original > Copy
I found that if you let it use the default "optimize" settings, it will corrupt old video.
Share > Export Using Quicktime
I found that if you use iMovie's export it reduces the quality of your video.
I followed youtube's advice for iMovie export settings.
Options > Video Settings Compression Type: H.264 Frame Rate: Original Key Frames: Automatic Frame Reordering: Unchecked Data Rate: Automatic Options > Sound settings Format: AAC Show advanced settings >Constant Bit Rate For size, selecting origial size didn't show the correct size so I used getInfo in finder to see the orignal dimesnions and set size to custome chosing those. Prepare for Internet Streaming: Fast Start