Imovie10 upgrade problem

After installing Mavericks I upgraded to Imovie10 which was a tortuous process. It looks like I have two sets of files as my spare capacity has reduced by half. If I look in the Events folders they contain .Mov files and when you try to open these they go through a conversion process. I assume these are Imovie9 files that I could delete. I have a large collection of videos and I don't want to risk losing any videos.