it wasn't straight forward, but using the information in this post:macOS High Sierra: IOConsoleUsers: gIOScreenLockState 3, hs 0, bs 0, now 0, sm 0x0 | tonymacx86.com I was able to complete the upgrade.
Basically I had to turn sip on using CSR 0X0, uninstall Nvidia drivers, reboot, reinstall...