I tired to update a record using save!. Mongodb_optimistic gem uses save and then update. Following is the output and still no updateExisting flag..
Please let me know if you need any other information.

MONGODB leaderboard_development['leaderboards'].update(

{"_id"=>BSON::ObjectId('4f3b08387877a646d400001e'), "_lock_version"=>1}

, {"$set"=>{"_lock_version"=>2}})
MONGODB user_store_development['$cmd'].find({:getlasterror=>1}).limit(-1)
lockable.rb:42: result

{"n"=>0, "connectionId"=>950283, "err"=>nil, "ok"=>1.0}