You shouldn't need to change all your existing queries, you should only need to change the logic that establishes your MongoClient. Most applications do this as some sort of Singleton so adding authentication is just a matter of modifying the Singleton. It is a pain-in-the-butt that there isn't a signature that takes just String, String for username password, but its the Mongo Java API, get used to disappointment.
You can either go the MongoURI path which gets you the shortest signature...
MongoClient mongo = new MongoClient(
new MongoClientURI( "mongodb://app_user:bestPo55word3v3r@localhost/data" )
);
Or go with the more verbose List<MongoCredential> path
List<ServerAddress> seeds = new ArrayList<ServerAddress>();
seeds.add( new ServerAddress( "localhost" );
List<MongoCredential> credentials = new ArrayList<MongoCredential>();
credentials.add(
MongoCredential.createMongoCRCredential(
"app_user",
"data",
"bestPo55word3v3r".toCharArray()
)
);
MongoClient mongo = new MongoClient( seeds, credentials );