(a) No health insurer shall, with regard to a group contract, 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 policyholder or group contractholder has delivered written notice of acceptance of the contract or policy.
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 or policy effective date.

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

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