All the offerings in the Windows Azure Store are online services, so the providers are responsible for any necessary updates. You are not required to take any action. However, this means the services you use from the Windows Azure Store are subject to change. As with any dependency you take on an online service, you should monitor for changes that impact your app(s).
If a user would like to upgrade to a different offer plan, the user can do so but take care as the upgrade will charge the user at the time of purchase and any unused functionality of current subscription will be lost. Otherwise, the user could purchase a completely separate subscription.