Core data custom migration

My old core data model has an NSDate field, which I would like to change to a NSNumber. I read the Apple documentation and several similar questions on SO and other blogs (see references at end of question)

But no matter what I do, I keep getting the same error:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Mismatch between mapping and source/destination models'

I only have 2 versions of the model, and I have verified time and again that the source and destination models are correct.

I even discarded all my changes and recreated a new model, mappings and entities (NSManagedObject subclasses). I've been stuck on this for almost 2 days now, and have no clue anymore as to what I'm doing. Any pointers on what I'm doing wrong will be greatly appreciated.

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator; 
    }

    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"Old.sqlite"];

    NSError *error = nil;
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];

    NSString *sourceStoreType = NSSQLiteStoreType;
    NSURL *sourceStoreURL = storeURL;

    NSURL *destinationStoreURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"New.sqlite"];
    NSString *destinationStoreType = NSSQLiteStoreType;
    NSDictionary *destinationStoreOptions = nil;

    NSDictionary *sourceMetadata =
    [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:sourceStoreType
                                                               URL:sourceStoreURL
                                                             error:&error];

    if (sourceMetadata == nil) {
        NSLog(@"source metadata is nil");