A quick example to migrate DBRef's to ObjectId's directly in MongoDB shell.

mongo_dbref_migration.js

/*
   A quick example to migrate DBRef's to ObjectId's directly in MongoDB shell.

   Suppose we have 2 collections:
   
   - db.authors
   - db.books, with books like {title: "My book", writer: DBRef("authors", ObjectId("anyID"))}
   
*/

// because of https://github.com/TylerBrock/mongo-hacker/issues/15
setAutoMulti(false);

var count = 0;

var process_field = function(x, fieldName){
   if(x[fieldName] && x[fieldName].$id){
      x[fieldName] = x[fieldName].$id;
   }
};

var process = function(collection){
  return function(x){
    process_field(x, 'fieldName');
    // here we can process as many fields as we want
    collection.save(x);
    print("Record #" + ++count);
  };
};

db.books.find().forEach(process(db.books));

// and this can be easily adaptable for nested fields

/*

  Rollback:

*/
var count = 0;

var rollback_field = function(x, fieldName, ref){
  x[fieldName] = DBRef(ref, x[fieldName]);
};

var rollback = function(collection){
  return function(x){
    rollback_field(x, 'fieldName', 'refName');
    // here we can process as many fields as we want
    collection.save(x);
    print("Record #" + ++count);
  };
};

db.books.find().forEach(rollback(db.books));