Does your customer have questions about upgrading to newest Windows server OS? Guidance on how to upgrade from one OS to another? What consideration needs to be taken before and after upgrading?
Send them to Windows Server Upgrade Center
When you upgrade a Windows Server in-place, you move from an existing operating system release to a more recent release while staying on the same hardware. Windows Server can be upgraded in-place at least one, and sometimes two versions forward. For example, Windows Server 2012 R2 and Windows Server 2016 can be upgraded in-place to Windows Server 2019.
The following graphic shows the upgrade paths for Windows Server. Downward pointing arrows represent the supported upgrade path moving from older versions up to Windows Server 2019. You can upgrade from an evaluation version of the operating system to a retail version, from an older retail version to a newer version, or, in some cases, from a volume-licensed edition of the operating system to an ordinary retail edition.
Reference: Windows Server Upgrade Center - https://www.microsoft.com/upgradecenter