It's funny because this game was designed in such a way that you are to play exactly as Squeenix wants and must not deviate from that path. Once you have access to a job you must never return to the base class.
Now you've found, if it's true and players will definitely find out, what could be a huge disruption to that design.
It's going to be fun to sit back and watch this unfold.
Will players abuse this? Will it become better to use the base class than the job 'advancement'? Will Squeenix allow this? Will they frantically try to fix it? How?