Upgraded four of my rigs thus far. One Ryzen desktop, I had some USB hub troubles, that led to a spontaneous reboot, in the middle of Windows Update doing the installation. It's also overclocked on air, which... sometimes... seems to cause issues with Windows Update? It was downloading... installing... download.... installing... it just seemed to keep starting from the beginning.
Before I started it, though, I had downloaded the ISOs and made a USB flash drive with 1809 (32/64 both).
So I used the flash drive to upgrade, and it worked, although it set my Radeon drivers back to "Graphics Mode", when I had them set to "Compute mode" for mining. I installed the newest AMD drivers anyways, in case there were tweaks for Win10 1809.
I also updated a Lenovo IdeaPad 110S Cloudbook. I had roughly 10GB free, and somehow, it upgraded, using an install USB. Then after it upgraded, I removed the old version of Windows using Disk Cleanup | System Files, and then I ended up with 13GB free. Pretty decent, really. Not nearly as difficult as I had it before, didn't get a free space warning.