Microsoft OEM Agreements, Software Maintenance, and Software Upgrades
Microsoft OEM channel use rights are a bit more involved than retail use rights, and Purchasing provides different ways to manage them.
OEM use rights for Windows (and no other products) usually grant the right to install and run the prior version of Windows. If you don't want to run the latest version of Windows, you can install the earlier version on top of it. Furthermore, an OEM license is tied to a single machine, and cannot be transferred to a new one.
If you enter into a volume purchase agreement with Microsoft, you can convert OEM software licenses to volume licenses for new machines that you purchase during the term of the volume agreement. Once the licenses are converted from OEM to volume, the provisions of the volume purchase agreement (for upgrades and downgrades) cover what once were OEM licenses. When you create a Microsoft volume contract using Purchasing, you can control whether you want to take advantage of this option. Converting licenses from OEM to volume does not involve any changes to the software already installed; it's simply bookkeeping.
If you do not have a volume contract (and, therefore, cannot convert your OEM channel purchases to volume), you should treat any upgrades using the same process as for Microsoft retail agreements.