.mov files will not play in QuickTime

I imported video clips from an SD card 5-10 years ago using iMovie's import media functionality. I now have Big Sur on my new iMac, and while the clips still play if opening the iMovie Library through iMovie, they do not play when opening the individual clips from the iMovie Events folder using QuickTime. They are .mov files, and I get the error, "This file contains some media which isn’t compatible with QuickTime Player." It then proceeds to open the files but will only play the audio. I want to back up these older clips to iCloud, but if the .mov files will not play, what is the point? Do I need to convert them somehow in order to back them up and play them on various devices in the future?