Ok, time to join CreativeCow, hello guys. I have this exact situation in front of me. I will have to experiment – I have tons of 1080 media and want a 720p project, especially since I want to reframe, zoom and stabilize. I would assume this works, but FCPX has let me down before. I will start with a project the size of 1080p and try exporting in 720p… not sure if makes sense though.