Like I wrote elsewhere, 1703 is by far the most 'smooth sailing' I've had upgrading whatever installation Win10-Win10 (and one 7-10 likewise worked perfect)
A few general hints though...
1. Uninstall any and all 3rd-party antivirus and other security minded real-time scanners and reboot before you begin... you don't want them installed and running (stopping them may not be enough)!
2. Download the upgrade and burn media/transfer to USB. This time I've mostly heard about freezing troubles from people doing online upgrades.
3. Don't rush it! On older installs, some steps seem to take a while... Worst so far is about 45 minutes for the middle phase (no hang, just a tad old hardware that's all)... In other words, take a walk or do whatever else and wait (especially frustrating on laptops without any storage activity led)
It behaves flawless on all but one install, but considering my hobby computer to be anything nearing normal would be a stretch and so it is to be expected. The fun now is that it does not happen too often and lasts for only a short time... so I have not been able to figure out what it is (I'm getting there)... The trouble? Let me show you a screenshot:
Now this here folder is usually sorted by 'Details' and did show only generic icons and as you can see, the 'View' tab has all view modes greyed out. Right-Click works though and that's what I did prior to taking the screenshot notice how there's no icons at all after changing to medium or large icons (not sure what I did choose), not even the left navigation pane show anything, no quick access a.s.o. 10-15 seconds later everything is O.K. again only to appear again from hours to a day later, this time affecting another folder. What puzzles me is that I can open other folders while this occurs and they display fine ('Launch folder windows in a separate process' active)... All I know is that it's related to explorer, but why not all?