Add new field to every document in a MongoDB collection
db.your_collection.update({}, {$set : {"new_field":1}}, {upsert:false, multi:true})
Remove field from every document in a MongoDB collection
db.your_collection.update({}, {$unset : {"old_field":null}}, {upsert:false, multi:true})
Rename field from every document in a MongoDB collection
db.your_collection.update({}, {$rename: {"old_field_name":"new_field_name"}}, {upsert:false, multi:true})