You don't need to downgrade. iMovie '11 (version 9) is still available to use and can be found in the Applications folder. You can still open the original Events and Projects in iMovie '11 and edit as before.
When you update Events and Projects a copy is made and placed in a folder named iMovie Library. The original files are untouched.
See these Apple articles:
Updating projects and events from old versions of iMovie -
Removing old libraries after updating -
I had problems with the new version, it removed all transitions in projects. When I replaced them it would not allow me to make the length of a transistion more than 2 secs so i have reverted back to the previous version of imovie following you advice thanks