The purpose of migrations is versioning of the database structure. The answer to any question similar to "where should I put database changes?" is always: "in a new migration", because then you're able to rollback changes.
In this case, I think I would first add Sentry 2 to your project and commit "Added Sentry 2". After, I would create a new migration with your desired changes, then commit: "Added fields x y and z to Users table".
See also the introduction paragraph of the documentation: http://four.laravel.com/docs/migrations