Hi
Adding or removing critical illnesses from the list of coverage would be a change to the terms and conditions of the policy. Hence it is more likely to be done for brand new business only, rather than for existing business (even with reviewable premiums). Bear in mind that CI policies are written not to pay out on diagnosis of any critical illness, but on diagnosis of any of the illnesses which are specified within the contract wording for that particular product.
So, for example, if the insurance company chose to add in one or more new critical illnesses to an in-force product and asked the existing policyholders to pay a higher premium as a result, this may not be considered to be fair. The policyholders might not want the coverage for those extra illnesses and so would feel unhappy that they were being expected to pay more for them - particularly when this is not what they had signed up for when they took out the original contract.
And if the insurance company chose to take away one of the critical illnesses from the list of covered conditions for an existing policyholder, they might become very unhappy if they then were diagnosed with that illness and the company would not pay out, despite their original policy document saying that this illness was indeed covered.
For reviewable premium business, it is normally the case that the premiums can only be increased due to a change in expectation of future claim rates for the cover as it is defined within the existing policy.
Hope that makes a little more sense?