What's your magic non-db level, supposedly-easier-than-updating-a-schema approach to renaming a field common to all existing documents in a collection, eg, rename an "author" field to "writer"?
PostgreSQL:
ALTER TABLE posts RENAME COLUMN author TO writer;
db.posts.update({}, {$rename:{"author":"writer"}}, false, true);
r.table('posts').replace(function(item) { return item.without('author').merge({writer: item('name')}); })