Hi, Keith,
iMovie 7 was a complete redesign and rewrite of iMovie 6, so it can't update or play the iMovie 6 projects.
If you can purchase the iLife 06 package from a 3rd party vendor, like ebay, you could download iMovie 6 from there and open your projects. That would be the best way to go since you are still running the 32 bit Mojave, and not Catalina. Take a look at this link:
https://www.ebay.com/p/127902544?iid=203246604097
Or maybe you still have your old iLife 06 Mac discs, and can download from those.
It is a little tricky, but fairly easy, to download iMovie 6. I am running it on Mojave, so it can be done. If you go the iMovie 6 download route, read through this link for a couple of ways to do it:
https://discussions.apple.com/thread/251451443?answerId=252822366022&login=true
-- Rich