Have you ever tried to block the upgrade on this computer? I ask because I know that some of us (including me) blocked the upgrade initially but had a change of mind later. The result in my case was that, although the folders containing the files were there, they were in fact empty.
The files for the upgrade download silently in the background automatically (unless you take measures to stop them). It might be worth looking to see if you have the necessary folders with the upgrade files in them. These folders are: $Windows.~BT and $Windows.~WS. They should be gigabytes big. It will be necessary to un-hide hidden files and folders in order to see $Windows.~BT and $Windows.~WS (Control Panel > Folder Options).
The trouble is that variables occur such as, what updates are downloaded/installed, whether the upgrade has ever been blocked etc.
In my case I had to go around a very long route but now, Microsoft allows you to use a Windows 7 or 8 key to do a clean installation Windows 10. I ultimately went for the clean installation because I think you end up with a cleaner and therefore more trouble-free system. That’s a matter of preference though.
Since Microsoft wants everyone to upgrade to Windows 10, they have brought out comprehensive instructions that they hope will cover all eventualities. I’ve read them and I think they do.
https://www.microsoft.com/en-us/software-download/windows10
If it was me, I’d click the ‘Upgrade now’ button and see what happens …but not before I’d backed up all files (documents photos etc). Preferably make a disk image of the entire system so that if things were to go pear-shaped, there is a way back better than using Microsoft’s own 30 day roll-back option. Success can’t be guaranteed.
The main thing to have if going for a clean installation is the Windows 7 key number.
However, if you are able to install the upgrade without going down the ‘clean installation’ route (called an ‘in-place’ upgrade), the key won’t be needed.
One thing that might be considered good (or creepy depending on how you look at it :)) is that if you do an in-place upgrade reasonably successfully and subsequently decide to do a clean installation because it is thought better, you won’t have to enter a key number (just skip that stage when asked for it).
The reason being that Microsoft will have profiled your computer on their servers. When you come to reinstall or install the OS afresh, the servers will know the computer qualifies for a Windows 10 installation.
These things can be a bit long-winded to describe. Sorry