Hi
Signing up for cover would allow you to upgrade, as well as accessing any other benefits included in the cover. It will be a minimum 12-month contract, after that it's up to you to continue or not. If you could send me a forum private message with your serial number, I'll be able to advise further. You can do so by clicking on my avatar, choose 'Send a message' from the top right.
v15 was developed way before Windows 10, Windows 8, or even Windows 7, we released v19.8 to be compatible with Windows 8, and v19.11 to be compatible with Windows 10. Since v15 is never tested on later Windows operating systems, or with later versions of Office, it may not work properly in that environment.
Have you also had an IT person to look into your old computer, perhaps to increase the hard drive space?