How to update iMovie on Mid 2009 MacBook Pro
I know this is not what you are asking however if iMovie will update to the latest version on your Mac there is a hack (that I used on Sierra) that lets you run the latest version of iMovie. I would make a copy of iMovie first, in case something goes awry. But this trick, worked for me.

It involves a simple script and some Terminal work but is not especially difficult, if you can get the current version installed on your Mac. 

But to be sure make a copy of your current version and if it updates, a copy of the new version too. If it goes south you can revert!