On Azure, I am currently using my \"3-month Free Trial\" subscription, and just recently I got access to a \"Windows Azure MSDN - Visual Studio Ultimate\" subscription.
There is no way you can accomplish in the Portal interface. You do have an option to request Windows Azure Billing Team and request them move all of your currently configured and running services from one Subscription to other subscription, the key is "all".
IF you want to move one specific service from one subscription to another then the available option is to deploy directly to other subscription.