I've posted previously about the projects I'm using FCPX for, and before I update to 10.2 I want to make sure it is fully compatible with 10.1.4 projects, and with primaries exporter...? Is 10.1.4 compatible with 10.2 projects?
After the upgrade to 10.2, your libraries will be updated for the new version - so no going backwards without backups. That said, I haven't had any issues going from 10.1.4 to 10.2. Well, I do have one issue with Pixel Film's LUT Loader, but they're looking into it.
Don't know about Primaries Exporter, as I don't use it.
Kind of impossible to tell but if you're really concerned you can check in with plugin makers to see whether your plugins are updated or functional. You could also create a new partition and update there and try 10.2 on a few projects while keeping a duplicate of the originals. I generally don't recommend update in the middle of a complex project as a rule. But personally I've had zero issues with 10.2.