Error: We couldn’t complete the updates, Undoing changes. Don’t turn off your computer.

I spoke with Level 2 support and told him all of the steps we had tried so that he would not waste another couple of hours retrying the same things.  He finally levelled with me that "the only thing that has been working for people who have this "undoing changes" problem with KB2919355 is to wipe your system and restart the update process from your original media.  

@RonSarid, I  am also experiencing supposedly successful installs on an x64 system followed by a rollback during the reboot. The last time I tried to get an update from Microsoft was not a pleasant experience - no-one had any new information but instead of getting that information for me insisted on taking over my laptop again.  Attempts to escalate to Level 2 were refused.  

I escalated through the Canadian Microsoft office and got a callback yesterday from the Microsoft Global Escalation Team.  The support person renamed c:\users\default, recreated the folder but was unsuccessful at getting KB2919355 to install.  Today I was informed that the best bet was an in-place repair - although the apps would need to be re-installed, user data would be unaffected.  I will be able to test this recovery method within the next week or so.  

My suggestion to those who are still experiencing issues is to follow the process (painful as it might be), escalate if necessary, ensure that Microsoft collects the required documentation using the Microsoft Support Diagnostic Tool (msdt.exe, requires a passkey provided by Microsoft Support) and keep escalating through whatever channels are available until you get what you need.

M$ doesn't appear to be releasing it manually though. All I can find on the official download channel are these two, both dated 5 May.

@Xircal, I wonder if the updates were only made to the Surface Pro version of the update.

Ron,

Are you getting any error codes which might provide a clue as to why it's failing?

Also, have you tried checking your hard drive for errors?

Here are the steps to take.

Right click the Start button, go to Command Prompt (Admin) and run the following command to check your hard drive for errors. If you have an SSD drive, run:

CHKDSK /f

If you have a standard rotary hard drive, run this command instead:

CHKDSK /f /r

You'll get a message that the tool cannot be run because the disk can't be locked and whether you want to schedule a diags check on the next reboot. Click "Yes" and then either reboot immediately, or finish what you're doing and then restart.

If CHKDSK finds bad sectors, it will mark them as such so that they can't be used. But if the tool appears to hang for more than a few of hours at a certain percentage, then that might  indicate that your hard drive is failing and will need to be replaced.

M$ doesn't appear to be releasing it manually though. All I can find on the official download channel are these two, both dated 5 May.

I forgot to mention, I then added Media Center, with the 8.1 Media Center product key, and that installed fine too.