Upgrading Windows Server 2008 R2 to 2016 sequentially

Our goal is to upgrade a machine from Windows Server (WS) 2008 R2 to WS 2016. For now, we don't want to perform a clean-install to the server OS.

But according to Microsoft documentation, there is no direct path to upgrade from WS 2008 R2 to WS 2016 but we can upgrade from WS 2008 R2 to WS 2012 R2, and then to WS 2016.

Since we are not going to use WS 2012 R2 at all, I was wondering if I can use evaluation versions of WS 2012 R2 and WS 2016 to sequentially upgrade from WS 2008 R2 to 2016, and then buy retail version (of WS 2016) to convert the WS 2016 evaluation to a retail version. Is this possible?

It is always possible to update evaluation version to retail using DISM cmdlet. According to the Microsoft document - https://docs.microsoft.com/en-us/windows-server/get-started/installation-and-upgrade

"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."

Also, you can check Server Role upgrade matrix here: https://docs.microsoft.com/en-us/windows-server/get-started/server-role-upgradeability-table

I once used retail versions to upgrade a 2008 R2 to 2012 R2 and then to 2016, that works. But I don't think you can use an evaluation version to upgrade a retail version at all. But you may not have to activate the 2012 R2 before moving on to 2016, so any 2012 R2 retail DVD may help.

This reason why you may want to upgrade rather than do a clean install, is that you have complicated third party software on the server that is very difficult or impossible to install on a clean OS.