Update: Finally got around to installing the update, that went well enough, but the Win10 install had its own updates, and wiped out my grub bootloader. I ended up having to boot from my arch install media (which I thankfully still had), in order to reinstall grub, reconfigure grub, and finally add the efi entry back. I'm now seriously considering removing Win10 altogether, because I almost never use it, and Win10 updates keep breaking my bootloader/efi.