WSL uses the lxrun executable for management from Windows:
lxrun -h
Usage:
/install - Installs the subsystem
Optional arguments:
/y - Do not prompt user to accept
/uninstall - Uninstalls the subsystem
Optional arguments:
/full - Perform a full uninstall
/y - Do not prompt user to accept
/setdefaultuser - Configures the subsystem user that bash will be launched as. If the user does not exist it will be created.
Optional arguments:
username - Supply the username
/y - If username is supplied, do not prompt to create a password
/update - Updates the subsystem's package index
Given that, you can use lxrun /setdefaultuser root. Just thought I'd point out this side of it since it was required when I ran into the same issue as you after trying to upgrade to Xenial. I can confirm that running this command, then the wget / dpkg commands my issues were resolved.
The commands I used:
wget http://mirrors.kernel.org/ubuntu/pool/main/s/sudo/sudo_1.8.9p5-1ubuntu1.4_amd64.deb
wget http://mirrors.kernel.org/ubuntu/pool/main/p/procps/procps_3.3.9-1ubuntu2_amd64.deb
wget http://mirrors.kernel.org/ubuntu/pool/main/s/strace/strace_4.8-1ubuntu5_amd64.deb
dpkg -i sudo_1.8.9p5-1ubuntu1.4_amd64.deb
dpkg -i procps_3.3.9-1ubuntu2_amd64.deb
dpkg -i strace_4.8-1ubuntu5_amd64.deb
Finally, you might need to run sudo apt-get install -f in case you get The following packages have unmet dependencies [xxx] but it is not going to be installed