6/21/2016 – Update, since I posted this article I have been told by a reliable source that there is a newer PowerShell script that does the job even more reliably. I haven’t tried it yet, but I trust the source and Microsoft Premiere Support directed him to this article. https://gallery.technet.microsoft.com/Azure-RM-Availability-Set-39e19d01
I was a little surprised to find out today that it is not easy to change what Availability Set a VM resides in once it is already created. The Azure portal has no mechanism of adding an existing VM to an Availability Set once the VM has already been created. Fortunately for me I stumbled upon this great resource.
By leveraging the PowerShell script available for download in that article I was able to add two existing VMs to an Availability Set that I had already created. What a life saver!