My SP3 wouldn't complete the upgrade, it kept getting stuck and then rolled back to Win8.1. Since I had partitioned the HD to run Win10TP on a 50GB partition, I copied all my files to that partition (or had them on mSD card or OneDrive), did a complete install from a recovery USB disk I made in Win8.1. As soon as it was up and running update said there were 391 updates to apply. I let it do that (took about 40 minutes) and as soon as that was done it said the Win10 upgrade was ready to install. It's been running great ever since.
As
@jnjroach points out, it's not an upgrade but a new clean installation with it then applying the user state. But I think letting it apply all the Win8.1 updates helped it be ready for the upgrade and having my user state be only the default install helped it give me a clean well running Win10. Sure I had to re-install all the apps I had and move files back to Documents, Desktop, etc. but my system runs great and I am having no problems like display driver crashes or BSOD as others have.