Upgrading iPXE booting Windows 7 to Windows 10

[-bat.]

Hi, I have a diskless machine which boots Windows 7 using iPXE and iSCSI. The iSCSi is served from a ZFS volume on FreeBSD and I am passing the options to iPXE using DHCP.
I have had this setup for a number of years and it works beautifuly.

Recently I tried to perform the free upgrade to Windows 10, but this fails to work.
the upgrade gets to the stage where the system reboots, but then does not perform the upgrade, simply gives me the option to go back to Windows 7. If I break out into a DOS prompt I can see the boot drive mounted as X:.

My suspicion is that the X: is the BIOS drive, and that Windows 10 has not understood the iSCSI bot from Windows 7 and has thus not located the boot drive. But thats just speculation really.

Has anyone else tried this and got it to work ? All help is much appreciated!