global class FooProvider {
    //V1
    global interface CustomFooProviderV1 {
        string bar();
    }

    public static CustomFooProviderV1 getFooProviderInstance() {
        // Use a CMDT or Custom Setting to get the name of the implementing class
        // Use Type.forName and Type.newInstance() to create instance
        //...
        return (CustomFooProviderV1)fooInstance;
    }
}

global class FooProvider {
    //V1
    global interface CustomFooProviderV1 {
        string bar();
    }

    //V2
    global interface CustomFooProviderV2 extends CustomFooProviderV1 {
        string raiseTheBar(int height);
    }

    public static CustomFooProviderV2 getFooProviderInstance() {
        // Use a CMDT or Custom Setting to get the name of the implementing class
        string className = 'localFooProvider';
        // Use Type.forName and Type.newInstance() to create instance
        //...
        Type t = Type.forName(className);

        // Create an instance to confirm the type
        object testInstance = t.newInstance();
        if(testInstance instanceOf CustomFooProviderV2) {
           return testInstance;
        }
        if(testInstance instanceOf CustomFooProviderV1) {
           return testInstance;
        }

        return (CustomFooProviderV1)fooInstance;
    }

    public class FooUpgrader implements CustomFooProviderV2 {
        private CustomFooProviderV1 v1;
        public FooUpgrader(CustomFooProviderV1 v1Param) {
            v1 = v1Param;
        }
        public string bar() {
            return 'foobar';
        }
        public string raiseTheBar(int height) {
            throw new InterfaceVersionException('Method raiseTheBar is not supported with an implementation of the v1 FooProvider interface');
        }
    }
    public class InterfaceVersionException extends Exception {}
}