It seems that one can only have one basic profession. But one can have multiple professions. And one can only advance from two professions at a time. This seems pretty straightforward.
But since one can only "buy" access to elite professions (since basic professions cannot be acquired beyond what you start with), does that mean that every time I want to change elite professions I have to spend a full advancement to get the new one and can no longer spend picks on the old one?
For example:
I have a starship officer. Her elite profession is Flight Control. She can buy professional abilities from both. So she wants to go into command. She spends an advancement on this and now can advance in Command.
Does she have to pick between Flight Control and Starship Officer as to which one she will also be able to advance? Say she chooses Flight Control. What happens when she wants to advance again as a Starship Officer? Since you can't buy Basic professions past the first one, what does this mean?