Open iMovie '09 .rcproject in iMovie 10

I have a vintage 2009 iMovie project (.rcproject) I rescued from an old Time Machine backup, and I'm trying to open it in iMovie 10 under Yosemite. It's large enough that I'm pretty sure it does actually have the video in it.

Unfortunately there's no just "open a file" option and everything I can find on line about the '09 -> 10 transition is about upgrading your iMovie "library". I haven't got a library, just one .rcproject file. How do I convince iMovie 10 this is something it can upgrade / open?

(Additional info: I do have iMovie 9.0.9 on this machine, but under Yosemite it refuses to launch.)