How designing a method doomed to change

I have the following abstract class for some plugin:

public abstract class BasePlugin
{
     public void SomeMethod(){..defaultBehaviour}.
}

This base class is going to be inherited from several (may be hundreds) of implementations.

I know that for sure, later I am going to change "in a small way" (but still changing) the behaviour of SomeMethod.

I would like the existing implementation of BasePlugin to continue behaving the same and the new one to use the new Feature.

Is there some pattern that allow me to do that ?

NB : I have the lead on all the implementations but I can not check for the hundreds of implementation if the new behaviour will be fine