Windows 10 Account lost after someone associated their Microsoft account to this computer.

Hi planck,
You got some good suggestions from both Mike and Neemo here to try. Since you asked several questions, there are several possible outcomes. But there are some things you left out that you may not be aware of I feel compelled to point out.

The question about reverting back to Win8.1 from W10 has a caveat there; and that is this can one be done within 30 days of your W10 installation date. If this is NOT the case, and you did your W10 upgrade more than 30 days ago, you have some other options I will outline for you.

Of course depending on whether or not you had previously created a Microsoft Cloud account *we call this a Microsoft Live ID*, this could go a couple of different directions. Let's say you didn't have a Microsoft account on your W10 computer or on any other computer. Neemo's suggestion would be a good way to go, as it may safely remove the Microsoft account from your W10 computer. From there you can perform a reversion back to 8.1--ONLY IF YOUR W10 UPGRADE WAS DONE WITHIN THE LAST 30 DAYS AS STATED ABOVE. If your local account is Ok after Neemo's procedure, and you did the upgrade within the last 30 days; you simply make the choice to stick with W10 or go back to 8.1. But you don't have to if you're ok staying on W10. If your local account is Ok, but your upgrade was done more than 30 days ago, read on.

If you get here, and your upgrade was done over 30 days ago, and you want to revert, there's no easy way to do it. (unless you happen to have a full Image backup using an approved Windows Image Backup software that's been tested with W10 such as Macrium Reflect, Acronis TrueImage, or EASETodo softwares). The reversion would require you to completely reinstall 8.1 from factory Recovery Media (if you have it-such as DVD discs or USB stick), or via a built-in factory Recovery Partition already on your computer's hard drive. We call either or these a Windows Reinstallation or Windows Reset. They are similar but operate slightly different and produce similar but not identical results.

Once you get your 8.1 reinstalled, and you use one of the above methods, you can use the new Microsoft account that you have created per Neemo's fix to login into the Microsoft app store. However, the Microsoft Cloud login doesn't work the same on an 8.1 computer as on a W10 computer, so it won't have complete access to the MS app store and all Microsoft services as it does on a W10 computer. We tested this during the Beta period of W10 (prior to July 29th 2015). This may still work for you. However, if you run into apps or games you can't access or purchase on you revered 8.1 computer, you may wish to run your W10 upgrade again and this time associate your new Microsoft account on your daily W10 login. **note: you can still create a W10 local account on your newly upgraded W10 machine if you wish, as on W10 you can actually have BOTH a local W10 login (no access to Microsoft app store), and a Microsoft account login (with full access to Microsoft app store). You just can't use them both simultaneously. You can only logon to W10 with one or the other [Local account vs. Microsoft account].

Finally, if you don't want to go back to W10 on that machine, you could always purchase a completely separate W10 computer/laptop/tablet, and keep 8.1 on your existing computer with or without a Microsoft account.

Hope this helps you.
<<<BIGBEARJEDI>>>