I've done my 3 desktops, my SP3, and my friend's Windows 7 laptop. The laptop hadn't installed updates since 2011(!?) so I had to spend a couple days updating it before I could start the Windows 10 upgrade. Went smooth though. My main desktop was having problems shutting down with fast startup enabled and I couldn't track down the problem, so ended up clean installing after a couple days. Luckily I was anticipating doing a clean install of Windows 10 on it anyway, so I didn't have very much installed on it yet. All are running like a dream now. Perhaps not coincidentally, my SP3 was the only device that upgraded right away through Windows update, the rest I didn't feel like waiting and used the Media Creation tool method.
Windows 10 won't accept your old Windows product keys. You need to install Windows 7, then do the upgrade. Then you can clean install to your heart's content after the key has been associated with the Windows 10 upgrade.
Will there be tweaks done after things are settled and done converting, etc? Things like showing the OP at the top of every page (I think this was in the last upgrade too and was removed) and the reply box saying "Answer this question..." What if I just want to join the discussion and not necessarily answer the question?!