Once you have a domain model with properly designed aggregate boundaries, you can transition it to the event-sourced model. Instead of modifying the aggregate’s data directly, model the domain events needed to represent the aggregate’s lifecycle.

The most challenging aspect of refactoring a domain model into an event-sourced domain model is the history of the existing aggregates: migrating the “timeless” state into the event-based model. Since the fine-grained data representing all the past state changes is not there, you have to either generate past events on a best-effort basis or model migration events.