At every even numbered level, the sorceror can choose to swap out one of their known spells of any level below the highest they can cast. So over the course of their career, they can swap out 10 of their spells. Same deal with the bard, only every level that is 2 mod 3 for 7 alterations to the spell list. From what I've heard, this has made them a bit better, since there's no need to be certain that every spell you take will be useful later in your career, e.g. sleep.