What are potential risks to upgrading software?

Well perhaps if you asked a more detailed question.... But in general if you upgrade an operating system, you could lose support for older hardware, because eventually developers stop writing the code for old stuff that needs to be replaced with something newer. For example I used firewire for many years because it was supposed to be better than usb. But eventually firewire disappeared and I had to switch my equipment to usb because no one was making the drivers for firewire in windows 10 or linux. Hope that helps.

For instance Konrad, many time hardware has it's own software, that requires a range of versions from anther software.

Real world example, I had an older PC that had a now older Nvidia graphics card. The computer was running Windows 7. Windows wanted to update to Windows 10, but the graphic card keep erroring out because Nvidia did not have software for that graphics card that was Windows 10 compatible. And Nvidia will never release an updated driver, as that graphics card is over 5 years old.

The risk updating to Windows 10, was that I was going to loose my support for my graphics card. Think kind of stuff is very common.

You could lose support or functionality of legacy software or hardware, such as old Windows or DOS programs, or firewire ports, stuff like that.