I have a car on a low mileage maintenance plan. The CBS will tell you when you have to come in for each service. Dealer will ask you for your key fob, they insert into their reader and the computer system will pull up any required maintenance. This record reflects any updates to the maintenace intervals from BMW. At that point, dealer will tell you what BMW will require them to provide to you for free. If you think other items needs to be serviced, it's up to the discretion of the service advisor to negotiate any other rate for these non-required services. The most common negoitation is on service that is slightly earlier than scheduled. For instance, I came in 15k before the required spark plug maintenace, and negoitiated an free early plug change in lieu of the original schedule.
so if you're not sure what can be done early, you need to at least go to the dealer and let them read your fob, as you car's CBS interval is not updated by BMW.