When migrating services in Azure from the Classic Model to the Azure Resource Manager (ARM) you might have some reserved IP addresses in your classic model. The ARM counterpart of the reserved IP is the public IP. Creating a new public IP will result in another IP address and might cause issues with clients that have whitelisted your IP address.
Although it's best to use whitelisting based on the FQDN. However, it is sometimes not possible and IP whitelisting is the next-best alternative. How do you migrate from a reserved IP address to a public IP address, without getting a new IP address?