You can change the premium mode if you are within your grace period from the policy anniversary date.
If you change the premium mode, then:
A fee will be levied on changing the premium mode
For monthly mode policies issued after January 2020, the grace period is capped at 15 days. For the remaining modes, the grace period stands at 30 days
For all policies issued prior to January 2020, the grace period is limited to 30 days
Note: In insurance policies where the term is more than 1 year, the policy effective date is considered as the policy anniversary date for the year that the policy is renewed.