Adding, removing or modifying something about an Object.
Definition
Change is a semi-structured way for players to alter something they've established about their Object.
It's structured because there is usually a list of things that they could Change.
It's unstructured because they can choose to Change multiple things, and the scope of the Change can be insignificant or monumental.
Players are asked to Change something about their Objectat various points in the game.
They can also choose to make a Change whenever they feel it's appropriate, to reflect the events of their story.
Change should be reflected materially in the players notes or Drawing—things are added, removed, modified, etc.
Intention
Change is a codified way to ask a player to reflect the consequences of their story on their Object.
The list of options for Change is usually anything that the player has committed to paper: Traits, Drawing, notes about their story, etc.
The feeling of loss when something is removed or destroyed is powerful, and can contribute to the themes of legacy and history. The player (and their story) is the only witness to what came before the Change.
You could just say 'change something about your Object', but by making it a choice from a structured list you have more control over the rate and nature of the Object's development. Players might also see options they hadn't considered as they look over the list, inspiring them to make a more interesting Change.
Example
In Artefact, players can choose from the following options when they're asked to Change something about their item (Object):
A detail about the physical appearance of their item.
A detail about something in their world—a name, faction, etc.
A response to one of their item questions.
Advice
If you want to impose more structure on the game, Change is a good place to explore this. You could make the options more specific in scope and intent, and explicitly limit players to a single Change when prompted.
Change often means that your Object is very different from the beginning to the end of a player's story. This is generally desirable, but you could minimise the role of Change in your game if you're more interested in how different Agents interact with the same Object.