I don't think it would be a good idea to allow people to change your advanced class. Bioware made the game around the idea that the player would have to make choices that matter. AC selection is one of these choices. How can you have a game built around choices that matter, when you can pay a few thousand credits to change your mind! This game is not wow. The class you choose at the beginning is not a class like you would find in world of warcraft, you make that final choice at level 10. They are not glorified specialisations, they are the second half of the class you chose when making your character! There is a reason they are called "advanced classes" instead of "specialisations".

Whilst I agree with your last point, I think you may misunderstand what the legacy system is supposed to be. Your legacy name is not your characters surname, though as you demonstrate, many people pretend that it is because you can choose to display it after your characters name. From what I have read around the subject it seems that bioware meant your legacy name to be a way of unifying your characters on a server under one name. If it was meant to act as a surname to your character, then why can you choose to display it underneith your characters name? The legacy system is mearly a tool bioware uses to encourage you to make alternate characters (much like the heirloom system in world of warcraft) without giving that character a combat advantage above those who have not unlocked it yet. Whilst it could be used as a roleplaying tool, it is not built to be one.