iphoto question

I have almost 40 GBs of little avi movies from my camera in iPhoto - and I know that if those were saved as mp4's instead, that I'd probably save a lot of space on my drives. But the problem is - there's no easy way, that I can think of, to take the movies I have in iPhoto now, replace them with newly exported mp4's and put back in the same spot retaining all their info. Does anyone have a
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