title: TypeORM migration timestamp type conversion
body: <p>I currently have a column entity in my postgres database:</p>
  <pre><code>  @Column({
      type: 'timestamp without time zone',
      name: 'appointment_date_time',
      nullable: true,
    })
    appointmentDateTime: Date | null;
  </code></pre>
  <p>And I would like to convert all the existing <code>'appointment_date_time'</code> field into <code>'timestamp with time zone'</code> in a typeORM migration. For example, the existing <code>'appointment_date_time'</code> field was <code>2021-06-24 11:00:00</code> (timestamp without time zone) would be converted to <code>2021-06-24 11:00:00.000-00</code> (timestamp with time zone).</p>
  <p>I tried this:</p>
  <pre><code>import { MigrationInterface, QueryRunner, TableColumn } from 'typeorm';
  
  export class appointmentDateTimeWithTimeZone1625773516977 implements MigrationInterface {
    public async up(queryRunner: QueryRunner): Promise&lt;void&gt; {
      await queryRunner.changeColumn(
        'case',
        'appointment_date_time',
        new TableColumn({
          name: 'appointment_date_time',
          type: 'timestamp with time zone',
          isNullable: true,
        }),
      );
    }
  
    public async down(queryRunner: QueryRunner): Promise&lt;void&gt; {
      await queryRunner.changeColumn(
        'case',
        'appointment_date_time',
        new TableColumn({
          name: 'appointment_date_time',
          type: 'timestamp without time zone',
          isNullable: true,
        }),
      );
    }
  }
  </code></pre>
  <p>But this overwrites all the existing 'appointment_date_time' data into null instead of converting them.</p>