Django - How change all values of a colum to null?

I have a problem, I make some changes in my models, because I had an Integer field that had to be changed to a ForeignKey

Well for error I assign to all the objects in the field derived_to the value 1.

I would now like to mark all the rows in this column as null. How can I make this? these are my last two migrations

class Migration(migrations.Migration):

    dependencies = [
        ('work_order', '0026_auto_20170603_1517'),
    ]

    operations = [
        migrations.AlterModelOptions(
            name='workorder',
            options={'verbose_name': 'work order', 'verbose_name_plural': 'work orders'},
        ),
        migrations.AlterField(
            model_name='workorder',
            name='derived_to',
           field=models.ForeignKey(default=1, on_delete=django.db.models.deletion.CASCADE, to='store.OutsourceOption', verbose_name='outsource'),
        preserve_default=False,
    ),
]

And the last one

class Migration(migrations.Migration):

    dependencies = [
        ('work_order', '0027_auto_20170605_0836'),
    ]

    operations = [
        migrations.AlterField(
            model_name='workorder',
            name='derived_to',
            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, to='store.OutsourceOption', verbose_name='outsource'),
        ),
    ]

Well thats all i need to make a new migration to assign null to all the objects in work_order in the column derived_to, because now all the objects have derived_to pointing to the first register on OutsourceOptions and that is not correct.

Thanks and sorry for my bad English.