db_change_field() is for changing the schema of a single column in a table, it doesn't relate to the field system as such.
The only way to change the name of a field is through direct (or pseudo-direct, using db API functions) manipulation of the database. These are things that need to change:
- Name of the field in
- Name of corresponding field in
- Name of the
- Names of the data columns in the data and revision tables (probably
FIELD_NAME_format in your case)
After that clear the cache (preferably manually, i.e. truncate all tables that match the name
cache_*) and you should see the changes in the site.
That should work on a stock install, but your mileage will likely vary if you have any contrib modules installed that provide functionality based on a field's name. I can't think of any except Field Collection of the top of my head, which won't apply to a text field, but there are probably others.
Whatever you do, take a backup of the database before you start playing around with it!