Ok, you can rename it but be prepared for weird bugs with Xcode 4.3 (and possibly earlier versions). In my case I created a new Core Data model file and added it to an existing project. Then I renamed the model file before attempting to build the project. Yet even with a clean build and install (deleting the app from the device first),
NSManagedObjectModel -initWithContentsOfURL returned nil even though my
NSURL pointer was non-nil.
NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"ModelName" withExtension:@"momd"]; __managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
Short answer: quit and reopen Xcode to fix the problem.
(Solution via Stack Overflow)