Modifications are typically aimed at increasing power, which increases an engines operating temperature and the amount of excess heat a spark plug needs to remove. While an unaltered engine will run best with the spark plug heat range originally installed, modifications such as adding a turbo or supercharger, increasing compression, timing changes, using alternate fuels, and sustained use of nitrous oxide may necessitate the use of a colder plug.
A good rule of thumb is, use one heat range colder for every 75-100hp added. When making heat range changes, it is best to err on the side of too cold a plug - running a plug that is too cold will cause fouling, whereas running a plug that is too hot may cause severe engine damage.
Caution: The heat range numbering system used by spark plug manufacturers is not universal. See the article, "What is a Spark Plugs Heat Range?" for more information on how the heat range works and how to select a colder plug.