Unfortunately there is no path to update iMovie 8. But, fortunately, the worst case is not a total disaster, because you still have the media and can redo your Dad's 60th Birthday Party video in iMovie 10 if that is necessary. However, there are some options that might consider that will let you recover your iMovie 8 projects.
First, open your more current iMovie 10 and do a File/Update Projects and events. It is possible that your iMovie 8 projects and events will update into iMovie 10 projects and events, where they can be edited and shared out as movies. It might not work since Catalina no longer supports 32 bit apps like iMovie 8. So the updating of Project and Events might not take place. Still, it is worth a try since it is so easy to do and might solve your problem.
If no luck, another thing that you can try is to use your Time Machine backup, if you have one, to revert back to your previous operating system where iMovie 8 worked. For instructions, go to the Finder Help menu and type in Revert to a previous OS. Or, you can reinstall a previous operating system using your Mac's recovery feature. For instructions, go to the Finder Help menu and type in Reinstall Mac OS. Always back up your system before restoring or reinstalling an operating system. If something goes wrong you don't want to lose all of your files. It is possible that you will lose some recently done work.
In the event that you do not wish to mess with the operating system, or if the procedure did not solve the problem, then you possibly might have a friend who is running El Capitan or an earlier operating system where iMovie 8 will work. You could place on an external disc formatted Mac OS Extended (Journaled) your iMovie 8 app and copies of the folders in the Movies file that are titled iMovie Project and iMovie Events. (Keep the originals intact.) If your friend will let you, plug in the external drive to your friend's Mac and open your iMovie 8 app on your friend's computer. You could then access the projects and share them out as movies to your external drive. Then plug in the external drive to your computer and access the shared out iMovie 8 movies.
As a last resort, you could open the previously made copies (keep the originals intact) of the folders from your Movies folder, titled iMovie Projects and iMovie Events. The Projects folder contains the codes for the project and the Events folder contains the media data. So, you should still have your iMovie 8 data intact in those folders, that you can import into the new iMovie 10 and reconstruct your project. If your old projects contain some legacy media, those would not work on Catalina, but you may be able to convert them to a more modern format using the free download, Handbrake.
For more guidance, you could contact Apple Support by clicking on the Contact Support item in the upper right hand corner of this forum's screen. Navigate to a phone number where you can call Support. The Support people are pretty savvy and might be able to assist.
Good luck with this.
-- Rich