To take matters even further, each of Kerrigan’s tiers includes two options- until you complete a particular mission on a particular planet in the campaign, at which point a third option is added to each set.
As with the non-mission-triggered choices regarding the various breeds of Zerg units, Kerrigan’s abilities can be reselected at any point between missions. At certain checkpoints in her level progression, Kerrigan will gain access to new ‘tiers’ of abilities, from which you may pick one to enable. In addition to these evolution options, Kerrigan herself comes with a wide array of options- as you complete missions she will gain ‘levels’ that increase her base capabilities, and there are optional side goals on many missions that will grant her additional ‘levels’. While some of these options are a matter of choosing between a more micro-management style and a more macro-management style, others can completely alter the overall purpose of a given unit. These opportunities come in two forms- each of the units affected has one option that you can reselect between missions as much as you like, and a second that is activated by taking a special mission while in the Evolution Pit- and cannot be changed for that instance of the campaign.
During the course of the missions in the Heart of the Swarm campaign, Starcraft II will provide you a number of opportunities to improve and alter your Zerg, customizing them to better fit your playstyle and preferences.