https://www.theregister.co.uk/2018/05/30/windows_server_new_previews/

One of the previews is for the Windows Server vNext Long-Term Servicing Channel, aka Windows Server 2019. The other previews the Windows Server Semi-Annual Channel, the version that gets a release every six months but is only supported for 18 months.

Both previews receive “in-place OS upgrade” from Windows Server 2012 R2, Windows Server 2016, meaning you can upgrade to the previews without a fresh install of the OS. Plenty of IT pros of The Register’s acquaintance are leery of such upgrades, preferring to start again from scratch.