Game concept
Reformation
The game also links it as Reform.
From the game's encyclopedia
The Culture Head is able to change certain parts of their Culture through Reformation. This is a process that takes a long time and can't be stopped once initiated.
The reformable parts are:
• Ethos
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Reformation is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Culture Head is able to change certain parts of their Culture through Reformation." The game also links it as Reform. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.