We've taken a very similar approach when migrating data from one DB to another (MySql to Redis in our case, but the principle should apply to any databases). We split it into 4 phases:

* Off - Data written only to MySql (starting state)

* Secondary - Data written to both MySql and Redis, and MySql shall be the source of truth.

* Primary - Data written to both MySql and Redis, and Redis shall be the source of truth.

* Exclusive - Data written exclusively to Redis.

As mentioned in the article, the Secondary phase allowed some time for the new database to be populated. And the distinction between Primary and Secondary phases gave us a rollback option if something went wrong.