The method describing how to change an `ENUM` in `Active Record and PostgreSQL` is a bit awkward. It put me on the wrong path for a bit. I found you can also change an enum by:
* renaming it
* renaming all the columns using it
* creating a new enum
* creating a new column using that enum
* do some copying of values.
Advantages:
* It runs in a transaction
* You can add/remove/rename enum values