code: def run_migration(data, version_start, version_end):
      
      items = []
      if version_start == 1 and version_end == 2:
          for item in data['accounts']:
              items.append(v2.upgrade(item))
  
      if version_start == 2 and version_end == 1:
          for item in data:
              items.append(v2.downgrade(item))
          items = {'accounts': items}
      return items
docs: Runs migration against a data set.