Thank you for the information and your question and I am sorry to read about your situation. Whether or not you have a claim against the STD/LTD carrier depends entirely on the exact language of the policy before and after the change. In other words, if the policy were to say, for example, coverage is established when the injury or illness is said to have occurred (or similar words), then you would have a good case against the carrier since the change in policy happened after your injury illness. On the other hand if it says a person becomes eligible under the policy for any qualifying illness or accident once it is diagnosed, then you would not likely have a claim since you were diagnosed after the change in the policy.
That said, it is true that the carrier has a duty to notify their policyholders of the change. However, whether or not a court in that situation would hold that since they didn't notify you, they couldn't apply the change against you, is not certain. Courts have gone both ways.
So, hopefully, your situation fits the first description. You will need to take all of your policy paperwork to a local attorney (ERISA specialist if this is an employer provided policy or a litigator if not) and have them review the facts and your policy information in depth to determine whether or not you will prevail in your appeal or suit. You do have a right to appeal the denial through the Company and then go from there.
Please feel free to ask for clarification if needed. If none is needed, then if you could take a moment to leave a positive rating in the box above, I will receive credit for assisting you today. Thank you.