Chicks need you to assist them in weaning off heat beginning at the end of their first week. As they grow in feathers to replace their down, they need approximately five degrees less heat each week. If chicks happen to be in a cool environment, they will feather out more quickly, and will need you to reduce the temperature of their heat source even more.
If you've been gradually reducing the temperature under their heat source, they should be ready to live without any heat, by the time they have completed growing in their feathers around four weeks as long as the ambient temperature remains constant. If you move them to a cooler environment, you may need to expose them gradually to cooler temps first. No extra heat is necessary, just acclimatizing.
However, if you haven't been reducing their heat as they grow in their feathers, they will require acclimatizing, gradual exposure to cooler temps while at the same time reducing their heat source.
To put it simply, abrupt temperature changes aren't a good thing. You need to do it gradually.