I'm trying to change my websites to the reserved mode but I can't... No matter the instance size I choose.
Update: Additional Capacity has recently been added: http://blogs.msdn.com/b/windowsazure/archive/2012/07/17/additional-reserve-instance-capacity-for-windows-azure-web-sites-preview.aspx
They are in the process of adding capacity all the time. I have run into his in the past as well. I know this is not the answer you want to hear, but try again every couple days and more instances will be available.
Keep in mind this is preview technology. Microsoft is in the process of assessing both usage and demand