First of all, you really do not need to change the IP address of any Windows Azure virtual machine. I will try to explain more about the details of the Windows Azure IP address.
Windows Azure Machine has a fixed IP address that is internal, and Windows Azure Service has a virtual IP address that is public. An internal IP address refers to each virtual machine (cloud service instance or virtual machine instance) that is assigned when the virtual machine instance is configured and remains unchanged for the duration of the virtual machine instance.
In the case of the Windows Azure Cloud Service Instance (PaaS), you have no control over the IP address of the virtual machine. However, if you are creating a Windows Azure virtual machine (IaaS), you can use the Windows Azure virtual network to combine all the virtual machines into a group of IP addresses / subnets, as described here . This may or may not be your requirement, however it is one of the ways in the IaaS VM, you can combine the virtual machine with your virtual network.

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