I don't know how to update codecs. I used System Toolkit to disable every non-Apple component, and the problem persisted. Although it was an Apple component, I disabled FCP Uncompressed 422. Then I had no problems opening the project in FCP, but it wouldn't let me open my sequence with that codec missing (even though there were no 422 assets involved in the sequence or project.)
Based on what I've read in other threads, I've also trashed FCP user preferences and repaired disk permissions.