Instance size flexibility for Azure Reserve Virtual Machine Instances is a new feature now available for all new and existing Azure RI purchases. Instance size flexibility can:
· Simplify the management of Azure Reserved VM Instances.
· Avoid the need to exchange or cancel your Azure RI to apply its benefit to other virtual machines within the same Azure RI VM group and region.
· Automatically apply Azure RI benefits that you have purchased to other VMs within the same group and region.
· Help you further reduce your costs in many scenarios.
This new feature applies to both Windows and Linux Azure Virtual Machines.
Source: Azure Roadmap