Object Oriented Design Pattern for Object Converter

I was wondering what the best OOP design pattern should be put in place for my scenario.

The scenario:

I have two classes, ClassA & ClassB. I want to convert ClassA into a new ClassB, however, ClassA may change its member variables & methods. This means that one may have to update the way ClassA is translated into ClassB.

What would be the best OOP design to go about facilitating this translation from ClassA to a new ClassB?

The goal:

Create a design pattern that will allow to easily change the way ClassA will be translated to ClassB.