Mavericks upgrade from Mountain Lion deletes all user accounts

Question

This is a lesson for upgrading too early (and too eagerly).
Today I tried the Mavericks upgrade from Mountain Lion on my 2007 Macbook Pro, after going through all the pre-installation/pre-upgrade checklists.
I don't think I have gone from a very high to a down low in such a short space of time.  I really wanted to move on from Mountain Lion and when the installer finished the upgrade to Mavericks and re-started my MBP, I was super excited.  When the restart completed and I was presented with the login prompt, I tried entering my user name and password.  First time, it shaked on the password... OK, you'd think that is a typo.  Re-entering the password... same thing.
You can imagine what ensued.  After entering my password v-e-r-y slowly many times, I started realising that something went very VERY wrong.
Just for information, I used two different user accounts on my MBP under Mountain Lion.  The one account had administrative priviledges and was only used for installing applications and a few other administrative tasks.  Then I have a second personal account, which does not have administrative priviledges and is the account I use daily for Mail, Safari, iTunes, programming etc.
None of those accounts worked under Mountain Lion.  Basically, there is no way in.
I then proceeded to boot with the startup option Cmd + S (command-line access).  Once that was completed, I was in as root and checked the /etc/passwd.  None of the user accounts were there.  I was only able to find two user accounts: root and nobody; and a long list of user account that started with an underscore (I think these are some sort of esoteric user accounts).  So, it appears that the Mavericks upgrade did not accurately bring over the user account information from my Mountail Lion installation.
Then, I went into the /Users directory and there I could see the home directories for all the users as they were defined under Mountain Lion.  The contents were intact too.
It seems to me that the Mavericks upgrade from my Mountain Lion forgot to bring over the user accounts but it did preserve the contents of all the users' home directories.
Unless anyone can spot some sort of user error here (please let me know), I am heading back to Mountain Lion pretty swiftly.  Otherwise, I suspect there is a bug in the Mavericks installer.
Any assistance is appreciated.

Answer 1

The reason this occurred is a mystery and has not been commonly reported. Consider the following. It may or may not help. You should probably print these instructions before proceeding.
Boot Single User mode again and do the following:
(You can skip the first step. It will check your boot volume for errors and correct what it can. If there were errors the installer should not have proceeded, but the possibility exists that directory corruption may have occurred during installation. That seems unlikely to me but could explain what went wrong.)
In Single User Mode:
At the :/ root# prompt, type fsck -fy and press Return or Enter. It will take a few moments to complete. If you are concerned that nothing is happening just press the Return key until you see the :/ root# prompt again.
Once the disk check is complete and the prompt returns, type mount -uw / and press Return / Enter.
Type launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist and press Enter. If you get an error message you may ignore it.
Type ls /Users and press Enter. All of the usernames on the computer will be listed, which is helpful if you don’t know or remember what these are.
Type dscl . passwd /Users/username password while replacing username with one of the users displayed in the previous step and replacing password with the new password of your choice. Press Enter. You will likely get an error message about a file or directory not being found. You may ignore this.
Type reboot and press Enter.