how to minimize number of classes to modify when interface is changed

There is an interface Accountable which has 2 methods. There are nearly 9 classes which implement the Accountable interface.

public interface Accountable{
    boolean isAccountable();
    float getWorth();
}

We have got a new requirement as follows: Two more methods declarations to be added to the interface. But we need to minimize the effect on the existing classes. I was told that we can use Adaptors to resolve the issue. But I am not sure how to do it. Could anyone please help me solve the issue?