can I downgrade FCP from 10.6.6 to 10.6.5?

MotionVFX hasn't said but I just ran into this problem with mObject 1.4.

I updated, accidentally, the last copy of the Library from before 10.6.5 to 10.6.10. Since it was just a portion of the production (one ending clip) that used the plug-in, there is a solution. Maybe this might help someone who is in this pickle in the future:

1. Export the project with the issue to XML with version 1.8. Note: Later FCPX versions might not be able to export to this version, so you might have to export a Version 1.9 XML than open with FCPX 10.6.9 or a few versions later like 10.6.10, and then see what made it through and then export to 1.8. Something like that, haven't tested this hoop exactly but it should be possible.
2. Then close the Library and quit that version of FCPX and open version 10.6.5.
3. Create a new Library and Import the XML. When you do, it should also import all the footage if you are doing this on the same drive and have to Copy to library storage location ticked in the Final Cut Import settings. It did for me.
4. Check your important clips with the plug-in in use. There might be missing clips and other stuff. Don't worry about it, we just care about the clips with the plug-in issue only.
5. Create a New Project with the same specs as your current imported one.
6. Copy the effected clips and the plug-in portion together (best to do one at a time unless you have a string of them together) and you can include what is around that clip if needed.
7. Paste the clip into the new project you created earlier, in the same Library as the imported XML one.
8. Check that they are the same with no timing changes or other issues. Fix any issues seen.
9. Export that as ProRes 4444 or higher (try for no loss of quality, depending on how important your video is YMMV)
10. Close the Library and quit FCPX 10.6.5 and open the new version. If you forget to close the Library, when the newer version of FCPX tries to open the 10.6.5 one, don't upgrade the Library.
11. Then Open the former upgraded library and import those exported clips and replace the pieces in the timeline with those video clips.
12. Now you can export the project again, the way it originally was or make changes with other parts of it that don't involve the plug-in pieces. Note: If you do need to make a change. Just go to your 10.6.5 XML imported Library with FCPX 10.6.5 and make the changes and repeat the process above.

Hope that helps anyone looking for a way out of this kind of situation. Coming up with this quick workflow, sure helped me, since I needed to export this older project same day!

This has been standard recommended procedure for all previous updates -- zip a copy of the existing FCP app (right-click and choose "Compress Final Cut Pro.app") before updating so you have this version to go back to if needed. I have not updated to 10.6.6 though so I don't know if once it's been updated it does anything that would prevent 10.6.5 from running properly or somehow break the Rosetta plugin mechanism. It shouldn't but hopefully someone like Tom or Luis can confirm.

Making a copy of a library to test with is a good idea. You shouldn't have to remove any of your existing libraries, just don't open them in 10.6.6. If 10.6.6 requires a new library format (most likely it does), it *should* ask when you open your test library if you want to update it. After being updated, this test library won't open in previous versions of FCP.

It does require a library update. The first version since 10.6.3.

Definitely be careful. 10.6.6 is incompatible with a number of plugins. Several FxFactory stopped working. Several MotionVFX plugins don't work. FilmConvert Nitrate doesn't work.

Be aware that if you use MotionVFX plugins, especially MO2, they will not be updating to 10.6.6. Instead they are working on the next full release versions to be compatible with Apple silicon.

How far away is that??