It's a bit of a clusterfuck [1] but you can buy a license to a newer version of Windows and exercise downgrade rights if you've got your old Windows 2000 media and product keys around, or something:
> It is also important to understand that although Microsoft grants downgrade rights we do not necessarily provide the means (media and product keys) to downgrade. So what does this mean? In order to downgrade software you will need the bits of the earlier version or edition and a product key. Microsoft only supplies bits and product keys to Volume License customers and only for certain versions and editions of the software. Volume Licensing customers have access to the Volume Licensing Service Center (VLSC) where they will have access to the current version and the prior version of the software they have licensed. We refer to the current version of software as version “n” and the prior version of the software as n-1. There are also certain core Microsoft products that we provide n-2 and occasionally n-3 versions of the software via the VLSC. Software titles where n-2 and n-3 versions are available can be viewed here. If you have legally obtained physical media for earlier Microsoft products that your organization is currently licensed to use through downgrade rights, you may use that media for downgrades.
And in fact, despite it being Windows Server 2000 you can exercise downgrade rights from Windows 10 Pro [2]. All the way back to Windows NT 3.51!
> It is also important to understand that although Microsoft grants downgrade rights we do not necessarily provide the means (media and product keys) to downgrade. So what does this mean? In order to downgrade software you will need the bits of the earlier version or edition and a product key. Microsoft only supplies bits and product keys to Volume License customers and only for certain versions and editions of the software. Volume Licensing customers have access to the Volume Licensing Service Center (VLSC) where they will have access to the current version and the prior version of the software they have licensed. We refer to the current version of software as version “n” and the prior version of the software as n-1. There are also certain core Microsoft products that we provide n-2 and occasionally n-3 versions of the software via the VLSC. Software titles where n-2 and n-3 versions are available can be viewed here. If you have legally obtained physical media for earlier Microsoft products that your organization is currently licensed to use through downgrade rights, you may use that media for downgrades.
And in fact, despite it being Windows Server 2000 you can exercise downgrade rights from Windows 10 Pro [2]. All the way back to Windows NT 3.51!
[1] https://blogs.technet.microsoft.com/licensing/2013/02/06/dow...
[2] https://download.microsoft.com/download/6/8/9/68964284-864d-...