What changes are to be managed when you are merging two organizations. If both organizations have a different culture and management approaches?

Culture and management have already changed the moment the merge has begun. First and foremost align the vision/objectives of both organization together. Then focus on getting people to believe in them. After that start look for a mutual benefits for everyone to cope with the change happen. you will have a lot of resistance and uncertainty in the beginning, but keeping the boat moving forward will create a healthy management and culture as long as everyone believe in the objectives.