Well, the Windows 10 April 2018 is here. In order to upgrade from the FCU to the new 1803 all you must do is go to Windows Update, and manually check for updates, and the new version will install.
Since lots of us X299 people had trouble upgrading from 1703 to 1709 is anyone here brave enough to try and update ? I think I am going to wait a few days to see how the upgrades go, but if anyone tries the update please let us know here how it goes. TIA