If the agreement is oral and 20 years old, it probably doesn't include provisions for termination, so who's to say what's "proper" notice? Consult a lawyer with experience in business transactions and, preferably, familiar with the insurance industry. If you cannot successfully negotiate with the new owner for a more workable time frame, an alternative would be to seek an injunction (court order requiring the other party to do something), but you will need to have a full and frank discussion with your lawyer to learn whether this route is likely to be successful.
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I am not convinced you have a verbal agreement. It seems to me that you have a course of conduct, e-mails and commission statements. These show an understanding of how a referred policy is going to be handled. This is not a 20-year old verbal agreement in my view. Remember that an e-mail and a commission statement is a writing.
Also this is not an agreement about future business, this is an agreement about policies already booked. I believe you are in a stronger position than you may think. If the company agreed to service a policy you originated, then they may be able to back out of that agreement, but I cannot think of a reason why they would be able to claim the policy as their own. if you didn't act fast enough.
Get an attorney if you can't arrange an extension after a few phone calls or e-mails.