Q: If I replace Mountain Lion with El Capitan will iMovies I have now remain in tact?

Hey there mr.greenfwhb,

I understand you have a question about upgrading to El Capitan. When you upgrade, iMovie will indeed update as well, and this article will explain how to update your projects so that they will be compatible with the new version of iMovie:

Update projects and events from previous versions of iMovie in iMovie (2014) - Apple Support

https://support.apple.com/en-us/HT202853

Thanks for using the Apple Support Communities!