With the release of Azure AD Connect v2, many Hybrid Identity admins find themselves with Windows Server 2012 and Windows Server 2012 R2 installations with Azure AD Connect v1 installations. As Azure AD Connect v2 does not support installation on these Operating Systems, Microsoft urges these admins to perform an Azure AD Connect swing migration.
Other admins may consider in-place upgrading their Windows Server 2012 or Windows Server 2012 R2 installation to Windows Server 2016 or Windows Server 2019, and then installing Azure AD Connect v2.
This is a bad idea.
What 'unsupported' means
In-place upgrade of Windows Server on an Azure AD Connect server is not supported. While you might pull it off, in the long term you might find yourself with an unsupported Azure AD Connect installation with the following consequences:
AUTOMATIC UPGRADES may be DISABLED
Azure AD Connect does not support Automatic Upgrades in unsupported conditions.
Previously, this measure was applied to Azure AD Connect installations running on Windows Server 2008 and Windows Server 2008 R2 and to Azure AD Connect installations running older versions of the .NET Framework. Now, you might find that your Azure AD Connect v2 installation no longer automatically upgrades to newer versions, after you in-placed upgraded the Operating System underneath it.
For many admins, Azure AD Connect is a 'fire and forget' type of installation. With Automatic Upgrades unavailable, you might see organizations stuck with troubled Azure AD Connect versions.
UPGRADES MaY BREAK
Automatic Upgrades may not be a problem. To go to newer Azure AD Connect versions, you can still perform manual upgrades. However, these manual upgrades may break at any point, too.
Performing a swing migration for Azure AD Connect is the preferred way to get to Azure AD Connect v2 when you're currently running Azure AD Connect v1 on Windows Server 2012 and/or Windows Server 2012 R2.