Scheduler ignores the changed start_date ?

[Ashika Umanga Umagiliya]

I used to have my start_date as '2017-03-14' for the DAG "cdna_daily_stg"
as follows :

default_args = {
    'owner': 'cdna',
    'depends_on_past': False,
    'start_date': datetime(2017, 3, 14),
    'email': ['some_email'],
    'email_on_failure': True,
    'email_on_retry': False,
    'retries': 5,
    'retry_delay': timedelta(minutes=5),
    'on_failure_callback': on_failure_callback,
    'on_success_call': on_success
}

dag = DAG(
    dag_id='cdna_daily_stg',
    default_args=default_args,
    schedule_interval="0 2 * * *"
)

Due to some code refactoring in my DAG, I wanted to change my DAG name to
'cdna_daily_stg_v2' so I changed by start_date as well to '2017-07-14' as
follows:

default_args = {
    'owner': 'cdna',
    'depends_on_past': False,
    'start_date': datetime(2017, 7, 14),
    'email': ['some_email'],
    'email_on_failure': True,
    'email_on_retry': False,
    'retries': 5,
    'retry_delay': timedelta(minutes=5),
    'on_failure_callback': on_failure_callback,
    'on_success_call': on_success
}

dag = DAG(
    dag_id='cdna_daily_stg_v2',
    default_args=default_args,
    schedule_interval="0 2 * * *"
)


But when I deploy my DAG with the new DAG , it keep starting DAG runs from
'2017-03-16' instead of from '2017-07-14'.

How to fix this ?
I still use same python file name, do I have to change the file names as
well as the DAG name ?