Were you leaving them alone waiting for something else? If no-one kills them... they don't spontaneously change. If they kill everything NOT a guardian, and leave the guardians... eventually it will be all guardians and stay that way until guardians are killed or the zone restarts for whatever reason (server down)
I did this once in Field of Bone. There were several different kind of Burynai in The Pit, but by selected culling I was able to breed an entire race of ...whatever type of Burynai it was.