Apparently I was wrong saying after last build upgrade (15042) that ESD to ISO
method seems to be dead. It is working again, build 15046 upgrade includes a normal install.esd
file if UUP has been disabled in registry settings. This time it's UUP to ISO
method that does not work; build 15046 upgrade using UUP method is OK and works as it should but UUP to ISO produces an error when writing ISO image.
In case you want an ISO and upgrade is not yet downloaded on your PC, edit the registry to disable UUP as told in this post
If the upgrade has been downloaded but you have not started the upgrade process yet (not clicked Restart now), you can remove the UUP upgrade files from your PC using Disk Clean-Up, edit registry to disable UUP, restart and let your PC download ESD upgrade instead. Instructions in this post
In case you have upgraded already and want an ISO image, the only option is to restore previous build, edit registry, remove UUP files as told above and start from scratch letting your previous build to download ESD upgrade.