title: Create new nested dictionary using nested key, values
body: <p>I have a dictionary of key-value pairs, where the value is itself a dictionary. I would like to change the names of key values in that nested dictionary based on a predefined conversion.</p>
  <p>I am using two lists to match up the values of the nested keys that I am trying to conver (<code>Old_item1</code> should become <code>New_item1</code>):</p>
  <pre><code>comparison_list = ['Old_item1', 'Old_item2']
  new_prods = ['New_item1', 'New_item2']
  
  old_dict = {
      'Company1':
          {
              'Old_item1':
                  {
                      'key1': val,
                      'key2': val
                  },
              'Old_item2':
                  {
                      'key1': val,
                      'key2': val
                  }
          }
  }
  </code></pre>
  <p>I tried this:</p>
  <pre><code>new_dict = {}
  for i in comparison_list:
      for j in new_prods:
          new_dict['Company1'][j] = test['Company1'][i] 
  </code></pre>
  <p>I get a KeyError: <code>KeyError: 'Company1'</code></p>
  <p>The desired output for each item I add to each list is:</p>
  <pre><code>new_dict = {
      'Company1':
          {
              'New_item1':
                  {
                      'key1': val  # old item key, val
                      'key2': val  # old item key, val
                  }
          }
  }
  </code></pre>