Categories for NSDictionary and NSArray for swapping the keys of a dictionary.

Swapping (key-newKey)

@interface NSDictionary (Swapping)

- (NSDictionary *)dictionaryBySwappingKey:(id)oldKey withKey:(id)newKey;

@end

@implementation NSDictionary (Swapping)

- (NSDictionary *)dictionaryBySwappingKey:(id)oldKey withKey:(id)newKey
{
    NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionaryWithCapacity:self.count];
    
    [self enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
        BOOL shouldSwapKey = [key isEqual:oldKey];
        if ([obj isKindOfClass:[NSDictionary class]]) {
            NSDictionary *newDict = [obj dictionaryBySwappingKey:oldKey withKey:newKey];
            id aKey = shouldSwapKey ? newKey : key;
            mutableDictionary[aKey] = newDict;
        } else if ([obj isKindOfClass:[NSArray class]]) {
            NSArray *newArray = [obj arrayBySwappingKey:oldKey withKey:newKey];
            id aKey = shouldSwapKey ? newKey : key;
            mutableDictionary[aKey] = newArray;
        } else {
            id aKey = shouldSwapKey ? newKey : key;
            mutableDictionary[aKey] = obj;
        }
    }];
    
    return [NSDictionary dictionaryWithDictionary:mutableDictionary];
}

@end

@interface NSArray (Swapping)

- (NSArray *)arrayBySwappingKey:(id)oldKey withKey:(id)newKey;

@end

@implementation NSArray (Swapping)

- (NSArray *)arrayBySwappingKey:(id)oldKey withKey:(id)newKey
{
    NSMutableArray *mutableArray = [NSMutableArray arrayWithCapacity:self.count];
    
    [self enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        if ([obj isKindOfClass:[NSDictionary class]]) {
            NSDictionary *newDict = [obj dictionaryBySwappingKey:oldKey withKey:newKey];
            mutableArray[idx] = newDict;
        } else if ([obj isKindOfClass:[NSArray class]]) {
            NSArray *newArray = [obj arrayBySwappingKey:oldKey withKey:newKey];
            mutableArray[idx] = newArray;
        } else {
            mutableArray[idx] = obj;
        }
    }];
    
    return [NSArray arrayWithArray:mutableArray];
}

@end