Hit up your MongoShell:
db.runCommand({
$eval : db.COLLECTION_NAME.find({
FIELD_NAME : {
$exists : true,
$type : 2
}
}).forEach( function(obj) {
obj.FIELD_NAME = new NumberInt(obj.FIELD_NAME);
db.COLLECTION_NAME.save(obj);
}),
nolock: true
});
All types have a numeric identifier and a full list can be seen here.
Much quicker than processing type operators on the application side. However, large collections will lock the database so your mileage will vary!