Right, I got this to work, but I'm not sure how. Some of the things I did: cleaned up the build folder, deleted all copies of the main application from my hdd, built a new release version of it, put it in Applications, picked that one when Xcode asked me to choose an app to run after building the thumbnail provider extension. That still didn't work, but afterwards I no longer got any store migration errors when building and running in Xcode. So basically the problem seems to have been that Xcode picked an old build of the main application to run with the extension, which created an old version of the Core Data store. Or the old store was still being cached somewhere. Just cleaning the build folder was not enough to fix this.

Thanks for clearing this up! Now let's see if I can make this work.