If you have a Linux machine or a Mac with MP4Box installed it's a simple command
MP4Box -inter 500 filename.mp4
This will shift the MOOV Atom from the end to the start of the file.
You can do the same on a Windows machine , there's a compiled Windows MP4Box I supplied to someone else from GFY who asked about this recently here:
http://adultking.co.uk/software/MP4B....6-rev2735.zip
Just open a command window and type
MP4Box -inter 500 filename.mp4
It will update the MP4 file so that it plays quickly with progressive download.