This can be achieved either with the web admin interface or using the API - MetaData. Every collection in database has a list of fields that are stored within documents in the collection. Not every document has to have every field. Each field has a data type defined for it: String, Number, Date, Names, Readers, Authors. Rich Text fields are treated as Strings, but return data in a slightly different format which is described in the Document API. List fields can be any data type, we handle that automatically for you.
To change a collection scheme you must be a super user. If you're using the web interface then there is a button in the title bar when viewing a collection "Modify Collection Schema", every field will be listed and you can change the data types for the fields. Similarly you can achieve the same thing by posting data to the metadata API service.
If you mark a field as Readers or Authors then document security applies on the document from the moment the schema is updated all future requests to access any document or collection of documents from the collection will have document security applied. So it is possible that a user who could see documents before will no longer be able to see them. Similarly if you remove Readers or Authors field definitions from a collection then document security will no longer be applied to the collection and anyone who has access to the database will be able to see all the documents in the collection.