If you try to build a CopyVisitor of some kind of object hierarchy, one has to build some kind of Stack structure, to give the actual visitor a hint, where it should write its copied data.

I suggest to extend all visitor methods with a return value:

MyType visitor(MyType param);

The standard implementation should return the actual object inserted.

public interface StatementVisitor {
    Comment visit(Comment comment);
    Commit visit(Commit commit);
    Delete visit(Delete delete);
    Update visit(Update update);