Good question. For your information, MongoDB stores documents in schema-less collections. As such, you really don't need to update the schema, but rather to update each document just as Peter Scheler has pointed out.
I.e. Adding a new field, use $set and then update the entire collection by db.collection.update()
However, reading all the documents and writing them back might take a great toll on your server. As such, versioning your documents would be a better alternative.