Current boss is selling the business I worked at for over 15 years! How do I deal with the switch over and not lose my length of employment there for saving my severance?

Whether or not your previous length of service is automatically recognized by the new owner will depend on the nature of the sale (asset vs share). It is very important that you do not sign any contract agreeing to waive your previous length of service. You should have your employer explicitly agree in writing that your previous length of service will be recognized.