title: Remove on update condition without drop mysql table
body: <p>I have a mysql table and data entries in it:</p>
  
  <pre><code>CREATE TABLE    `invoices`.`invoices` (
      `InvoiceID` bigint(20) unsigned NOT NULL auto_increment,
      `UserID` int(10) unsigned NOT NULL,
      `Value` decimal(10,3) NOT NULL,
      `Description` varchar(4048) NOT NULL,
      `DateTime` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
      PRIMARY KEY    (`InvoiceID`)
  ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=latin5;
  </code></pre>
  
  <p>I want to remove "on update CURRENT_TIMESTAMP" condition. How can I alter this table?</p>