title: Pyspark changing type of column from date to string
body: <p>I have the following dataframe:</p>
  
  <pre><code>corr_temp_df
  [('vacationdate', 'date'),
   ('valueE', 'string'),
   ('valueD', 'string'),
   ('valueC', 'string'),
   ('valueB', 'string'),
   ('valueA', 'string')]
  </code></pre>
  
  <p>Now I would like to change the datatype of the column vacationdate to String, so that also the dataframe takes this new type and overwrites the datatype data for all of the entries. E.g. after writing:</p>
  
  <pre><code>corr_temp_df.dtypes
  </code></pre>
  
  <p>The datatype of vacationdate should be overwritten.</p>
  
  <p>I already used functions like cast, StringType or astype, but I was not successful. Do you know how to do that?</p>