I am not happy. This is not a simple update, but something like the upgrade from W7 or W8.
It may take hours. What are the risks, what may go wrong...
If they do this regularly, to all their users, who understand hardly what is going on, to me it looks like a very bad idea....
Just my thoughts.....
My dual boot W7/W10 (i7-2600, GTX560Ti, 8GB, W10-64 Pro on SSD, W7 with 'My whatever' on HD) went OK, it took less then an hour
My (very small, maybe even too small - but it is a nice test configuration) Acer One (N270, 1GB, Intel 945, HD 148GB with W10-32 Home) went OK, it took 3 - 4 hours
Being an insider I've had it for a couple of days. Hasn't broken anything and the update didn't take long at all. Though obviously peoples experience my vary. I do presume though if you have a stable 10 the upgrade will be painless.
It took me around an hour and half or a little better. It is up and running good so far. Went to upgrade a Toshiba laptop and it crashed on me. That was a Windows 10 Home version. Now I am in the process of doing a clean install. It is a mess.
My thought is that it won't be a bad idea, when you could postpone these, what I consider to be 'high risk', reinstalls of Windows, until you have made the precautions you thought to be necessary, like freeing disk space and making a system copy.