Notwithstanding some of my earlier comments in this thread, it occurred to me that I have an HP laptop, which I bought for extended trips but which is rarely used these days. It has Win-7 installed which I didn't like much – probably because I never spent the time to chase down and change all the MS dumbed-down, self-serving and over-cautious default settings. I thought I could do worse than experiment with W-10 on this machine. So…
1. Arguably the simplest way to get around the "will it screw up my computer?"; "can I undo the upgrade?"; "how long do I have to do that?" type questions is to back up the Win-7 system first. I elected to back up the whole HD. First I tried doing (selected) outstanding Windows Updates on the W-7 setup but found, as others have, that I couldn't actually make contact with the MS server. So I decided it didn't really matter and went ahead with the backup anyway.
(As an aside, it seems the MS servers are getting pretty busy these days. This might explain MS's policy of forcing automatic updates on users, since by that means it can control the timing of traffic to its servers. Just a thought).
2. By going to **LINK** I found the confirmation of my earlier head-scratching post – that you can actually download an upgrade image file to write to DVD or USB-Drive (whether downloading and saving it would protect you when the free upgrade offer expires in June – or whenever – remains to be seen …. the image might expire at the same time). I downloaded the DVD image and cut a disk.
3. It's not actually very clear whether the install disk thus created is supposed to be used as a boot disk or whether the setup is to be invoked from within Windows. I tried booting with it and it asked me a bunch of setup questions then told me that it couldn't proceed until I started Windows and ran setup from there (!). So I did that and it re-ran the install but, significantly, didn't repeat the questions that it asked before. So it seems at least that smart.
4. Somewhere along the line (on the website I think) you are admonished to be sure to remove the DVD from the drive before the first reboot. Otherwise the setup will start over I didn't see any warning of the reboot and more or less guessed when would be a good time. Considering that some of the process is very lengthy, there is no pause button – and some of us need the bathroom! MS could do better here I think. This probably wouldn't be a problem if you used the alternative USB drive install (unless, presumably, your machine is set to boot from USB).
5. I made sure not to accept any defaults throughout the setup but went the "customised" route. In almost all cases that resulted in rejecting the MS defaults.
Other than that, the upgrade seemed to go fairly well. I haven't actually used it significantly since. I did install Classic Shell (wouldn't want to run any post-XP version of Windows without that). I dug around the internet and have a list of the recommended settings to change from MS defaults – mostly concerned with privacy. Most of these I'd already covered by not accepting the defaults. Now I just face the uphill battle with MS's over-the top security that prevents me from accessing a whole bunch of MY OWN HD. That could take many weeks if the Win-8 experience is anything to go by (I won't be updating that machine any time soon).