My PC upgraded a few days ago, apparently successfully. It did change some of my privacy settings but I soon changed them back (not that I'm complacent - I'm sure we'll never know just how much
they can find out about us).
For anyone in doubt the upgrade does a full re-installation, just like the original upgrade from Windows 7/8, but leaves all your program and data files intact as far as I can tell.
I have disabled just about all the
special features of Windows 10 and use a third party browser, email client, etc. In this state it is hard to tell it isn't Windows 7

Mike.