In case anyone is in the same boat as I am, I'll tell you what I did, since there were no replies to this post. Phone calls to Apple support aren't as fruitful as they once were. I can recall several years ago calling about a similar issue and was connected with an iMovie specialist who knew the app well. Now when you call, you get a gatekeeper who can't answer your specific question - instead of connecting you to someone who can, that person puts you on hold and asks someone. I was told that yes, I could update to Big Sur and that iMovie SHOULD work. They didn't sound too sure. Hey Apple - this new method does not engender confidence. Please go back to what you did before. Anyway - I ended up finding an outside person online who was a little more knowledgeable about my question. So here is what I did: I copied the iMovie Library file where all of the data for all of my iMove projects are kept. Then I copied again to another external drive. Next, I update to Big Sur from Catalina. When I booted up, I noticed that the iMovie icon in my dock had a slash through it, meaning it wasn't compatible with OS 11. So, I went to the App Store to download the latest iMovie -there was a note saying "this version of iMovie not compatible with your OS" I started to panic. Looking in the communities posts, I found someone who said to drag the iMovie app into trash - but NOT to delete it. Doing that, changes the prompts in the App Store listing for iMovie (now, a cloud symbol appeared which allowed me to download a previous version that would work with OS 11. It took quite a while for that to download and install. I was sweating it out, believe me. Once done, I launched it and it found my iMovie Library file from the previous version and updated that, praying nothing would go wrong in that process. The prospect of losing all that work brought on dread like you read about. The process looks like it worked. I was looking at the timeline where I left off in iMovie 10.1.12. I will do a deep dive tonight just to make sure everything is working. Wish me luck.
