gens don't change, but the stage an evolving item is at will change periodically until it gets to the final stage when all the options become available.
gen stands for generation, which indicates a fixed position (generation 1 will always be generation 1, 2 will always be 2, and so on, regardless of what stage of life they're at). it's like trying to figure out your family with your grandma and grandpa as the first generation, then your parents as the second, you as the third, your kids as the fourth, and so on. you don't magically become some generation other than the third once you move from childhood to adulthood, do you? you just change which stage of life you're at.