(a) 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 in subdivision (b), during any of the following time periods:

1. (1) After the group contractholder has delivered written notice of acceptance of the contract.
2. (2) After the start of the employer’s annual open enrollment period.
3. (3) After the receipt of payment of the premium for the first month of coverage in accordance with the contract effective date.

(b) Changes to the premium rates or applicable copayments or coinsurances or deductibles of a contract shall, subject to the plan meeting the requirements of this article, be allowed in any of the following circumstances:

1. (1) When authorized or required in the group contract.
2. (2) When the contract was agreed to under a preliminary agreement that states that it is subject to execution of a definitive agreement.
3. (3) When the plan and contractholder mutually agree in writing.