You can recover the original Core Data model from the production app's bundle.
Use Finder to open the app bundle (Show Package Contents) and look for a .mom file or a .momd directory.
Copy the file or directory to some location outside the bundle.
Create a new XCode project with Core Data.
In XCode select the newly created projects Core Data model.
Go to the XCode Editor -> Import menu and select the .mom file you have just copied, if need be find the correct version in the .momd directory.
You should now have the model in XCode, save and copy the model file to you original project...
In XCode delete the incorrect model from your project (remember to save a copy first so you have a copy of the new changes)
Use the XCode Add File to... menu to add the old model to the original project
Now compile and run the app to confirm it opens the old file correctly
Now add a new model version and add the changes to this new version - make sure you select the new model version in XCode before making changes