After you have created a subscription, you can easily change the plan and add-ons associated with it.
1) Go to Subscriptions.
2) Click the subscription.
3) Click Change Plan.
4) Choose the new service plan from the menu. If you select "None", the subscription will be unbound from its plan and you will be given the opportunity to create a completely custom set of resources and services instead.
5) Add or remove add-ons here.
6) Click OK.
That's it! Now you know how to change a subscription's plan and add-ons.