How to extend builder class in Java

I want to extend my builder pattern class. below is my existing class.

@JacksonRootElement(localName="Test")
@JsonDeserialize(builder = Test.Builder.class)
public class Test{

    @JacksonXmlProperty(localName = "TRAIN")
    private final List<Train> trainDetails;

    @JacksonXmlProperty(localName = "NAME")
    private final String name;

    @JacksonXmlProperty(localName = "DETAIL_TRAIN", isAttribute = true)
    private final Optional<String> detail;

    private Test(final Builder builder) {
        ///field declaration
    }

    public static Builder builder() {
        return new Builder();
    }

  //getters, hashcode and equals, toString

    public Builder toBuilder() {
        return builder()
            .withTrainDetails(trainDetails)
            .withName(name)
            .withDetail(detail.orElse(null));
    }

    public static final class Builder {
        private List<Train> trainDetails = ImmutableList.of();
        private Optional<String> name = Optional.empty();
        private Optional<String> detail = Optional.empty();

        private Builder() {
        }

        //builder methods

        public Test build() {
            return new Test(this);
        }
    }
}

Also my train class has like below:

public class Train {
'
    private String number;
//builders
}

Here I want to create a new class with same properties in Test class and only change in Test class is @JacksonRootElement is different @JacksonRootElement(localName = "Test123"). Also in Train class I want to extend it with new field code instead of number.

Anyone please help me to achieve this