Will iMovie 9.0.9 still run ok when using Yosemite

Yes, upgrade your iMovie 9 to the last version (9.0.9) before updating the OS. (I think you meant to say you were using an earlier version than 9.0.9). If you upgrade the OS over your Lion system, iMovie 9.0.9 will be moved into its own folder in Applications and should still work OK, though a few have reported problems. Don't do a 'clean' instal of Yosemite else you will loose it.

Geoff.

Sorry I slightly misread your original post. iMovie 9.0.9 should work fine in Yosemite and you will have option of updating your projects into iMovie 10 which will appear in your applications menu.

Geoff.

Be careful switcher, I tried to import my iMovie 9 files into iMovie 10.0.9 and it doesn't work. The Genius Bar couldn't help and now I think my best option is to either find a computer with Mountain Lion and iMovie 9 and generate movie files with all my incomplete projects and then bring them over.
So no, iMovie 9 is not compatible with iMovie.

" I tried to import my iMovie 9 files into iMovie 10.0.9 and it doesn't work."

By iMovie 9 do you mean version 9 or iMovie '09 (version 8).

iMovie 9.0.9 (the last version of iMovie 9) projects and events should import into iMovie 10 with no problem. Whether you would find iMovie 10 an improvement over iMovie 9 is another matter but you can try it out since importing iMovie 9 projects does not stop you continuing to use iMovie 9..

Geoff.