title: Migrating Primary Key to use AUTO_Increment
body: <p>I have a table where the primary key is sale_id, payment_type</p>
  
  <pre><code>mysql&gt; describe phppos_sales_payments;
  +----------------+---------------+------+-----+---------+-------+
  | Field          | Type          | Null | Key | Default | Extra |
  +----------------+---------------+------+-----+---------+-------+
  | sale_id        | int(10)       | NO   | PRI | NULL    |       |
  | payment_type   | varchar(255)  | NO   | PRI | NULL    |       |
  | payment_amount | decimal(15,2) | NO   |     | NULL    |       |
  +----------------+---------------+------+-----+---------+-------+
  </code></pre>
  
  <p>I want to add a new field <code>id</code> that is an auto increment. The reason for this is I want to track payments of the same type individually. Is it possible to do this without losing data?</p>