I agree with Lyrexio, this sounds way better than talking to a NPC and simply buying your new/better skills with money or in this case to get a different subclass.
I also prefer to have a connection to my character on which I am going to spend a lot of time on. And having to achieve stuff to actually improve my class/walk a different path is establishing a deeper connection to your character. As long as you "are into that stuff" at least.
Edit: It can be annyoing, especially for those who don't want to progress through a lot of stuff before they can access new classes. To counter the problem of having to create a new character because you didn't know about the different subclasses you can have a small tutorial or just a NPC which tells you which path you can choose with your current class.