Now, converting integers to strings will always work. But, you feel it coming, converting a string into an integer will not always be possible. In other words, we can't reverse this migration.
That's why we should raise ActiveRecord::IrreversibleMigration in the down method.
Now, if you run your migration (upwards), you'll see it being applied like it shoud. However, if you try to go back, you'll see rake aborting with ActiveRecord::IrreversibleMigration.