No group health care service plan shall change the premium rates or applicable copayments or coinsurances or deductibles for the length of the contract, except as specified. A change in premium rates or changes in coverage stated in a group health care service plan contract shall not become effective unless the plan has delivered in writing a notice indicating the change or changes at least 60 days prior to the contract renewal effective date. For small group health care service plan contracts, if the department determines that a rate is unreasonable or not justified consistent with Article 6.2 (commencing with Section 1385.01), the plan shall notify the contractholder of this determination.