我认为，最好分四步来做：
1.首先添加包含NULL的字段assignedtip_date作为开始;
1.然后运行 * 数据迁移 *，用assignedtip的截断部分填充字段;
1.然后删除assginedtip字段;和
1.最后将assginedtip_date字段重命名为assignedtip。
因此，迁移看起来像：

# app_name/migrations/0123_some_name.py

from django.db import migrations, models
from django.db.models.functions import TruncDate

def migrate_date(apps, schema_editor):
    MyModel = apps.get_model("app_name", "MyModel")
    MyModel.objects.update(assignedtip_date=TruncDate('assignedtip'))

class Migration(migrations.Migration):
    dependencies = [("migrations", "0001_initial")]

    operations = [
        migrations.AddField(
            "MyModel", "assignedtip_date", models.DateField(null=True)
        ),
        migrations.RunPython(migrate_date),
        migrations.RemoveField("MyModel", "assignedtip"),
        migrations.RenameField("MyModel", "assignedtip_date", "assignedtip"),
    ]