About June I think.
You will be able to update Windows 7 or 8 to Windows 10.
It will simply be a large update by normal Windows Update system rather like XP SP3 for example.
It won't be like a re-install - everything should be there and work as before, although there's probably more chance of issues on older machines on W7.

It's likely similar machines pre-installed with W8 will be discounted compared to W10, I think we see it already.
Windows 8 is different but not thast bad especially now on 8.1
This guide runs through steps to make Windows 8 PC behave more like 7 (or XP).
https://www.thurrott.com/windows/windows-8-1/159/make-windows-8-1-look-work-like-windows-7

When you first boot a W8 computer and it pops up the tiled metro screen it is a bit of a shock.