I experimented a bit and found out that if you save a story sim to your library and load them back into CAS later, you can edit their traits and aspirations, but the preset job, skills etc. are gone - or at least you can no longer view them. This means this feature is only useful if you create a sim from scratch and start playing with them right away. Makes it even less useful for me, sadly. (And it makes me wonder how exactly those new cover sims are going to be implemented, I assumed they would come with preset skills and jobs as well.)
It can come up with some funny results, though. The first (and only) sim I created this way got the Good trait, but also the Public Enemy aspiration!