The Law of Consonantal Shift was a principle that was developed by the Brothers Grimm. The law proposed that language changed over the course of centuries as words changed in a consistent fashion.
The Fourth Doctor mentioned this to Romana II when discussing how the names of the Hydrax's officers, Miles Sharkey, Lauren MacMillan and Anthony O'Connor, had changed to Zargo, Camilla, and Aukon. (TV: State of Decay)
Bernice Summerfield told Keri that, according to the Law of Consonantal Shift, the word "Agravan" — as in the Lost Mudfields of Agravan — would have changed to "Caravan", but she soon admitted she had made it up on the spot and couldn't really remember the real details of her discovery of the Mudfields. (AUDIO: Buried Treasures)
When Paradise Towers fell into ruin, the names of some of the groups mutated. The gangs of girls who roamed the corridors became known, even to themselves, as Kangs and the residents became know as Rezzies. (TV: Paradise Towers)