The devs actually discussed this in the last Q&A. Short version, they have no interest in implementing this feature at this time. In other words, the answer is a resounding no. To be honest, I don't think they should allow it anyways simply because they are two completely different classes. I have two Jedi knight characters. One is a Sentinel, the other is a Guardian. They are vastly different class types and it wouldn't make much sense to change your advanced class after you've chosen.