Copying a value to another doc field in one line

To do a backup inside the same collection document I do:

_.each(MyCollection.find({something: true}).fetch(), function(doc) {
  MyCollection.update({doc._id}, {$set: {backup: doc.content}});
});

Is it possible to copy a value to another doc field in one line without fetching the document first ?
Example:

MyCollection.update({something: true}, {$set: {backup: $content}}, {multi: true});