Taken from the OU FAQ section...
"Can I change my course or module?
You can change your course or module to another, at any time before the start date of your original course or module, as long as there are enough places on your new choice of course or module. If there is a difference between your original fee and the new fee you will be required to pay this before we can register the change. If your new course or module is cheaper than the original, you will receive a full refund of the difference.
If you want to change after your course or module start date you may be entitled to a discount on the fee of another course or module, as long as it will start within 18 months of your original start date and the fee has been paid in full at the date of the transfer. You will not be entitled to a discount on any course or module if the date of your withdrawal is on or after the first day of any residential school that you must go to. Short courses or modules do not qualify for any such discount.
If you are thinking about changing or withdrawing from study, please telephone us as soon as possible on 0845 300 60 90 to talk through your options."