Assign static (reserved) public VIP to existing cloud service/VM

Azure introduced the static VIP option, great!
I've read the documentation on this: http://msdn.microsoft.com/en-us/library/azure/dn690120.aspx

However, this only explains on how to create a NEW cloud service/VM with a static / reserved IP.
I already have a few services running. How can I assign a static/reserved IP to an existing cloud service?
I can't seem to find any documentation on how to do this.

Any tips/advice?