It's easy to change your membership plan from the All Access to the MSN Download Game Discount Program (or vice versa).
In order to do this, cancel your current subscription from within your account pages. After cancellation, you will have the option to re-start a subscription. Select that option and choose the subscription plan you wish to have.
Note that your monthly recurring billing date will change to the date your new subscription plan is initiated. Example: Today is September 10. If you are a current MSN Download Games Discount Program subscriber, whose rebilling has traditionally taken place on the 5th of each month, and you switch plans today you will be billed today and your new rebilling date will be on the 10th of each month going forward.