Are you able to go back and change all your advancement changes once as you level up to a new advancement choice or are they still locked?
Choices that you make are locked.
Edit:
Just to be sure you understand everything, I’m referring to class choices only.
You can still go to the class masters of your previous rank class and train attributes and change on/off settings for certain attributes that have to option to do so.
You just can’t change your class choice, once you make it.