If you do that, the first thing it does is ask for an activation key. Your windows activation key from your original Windows media is likely to not be accepted. My 8.0 key wasn't.
Had the same thing. I found out that you have to use the same SKU type for windows 10 as your original OS. So if you had windows 7 home, you can't upgrade using the windows 10 pro iso, you need to install windows 10 home. Also, not every key seems to work. All my MSDN keys failed, and also a windows 7 pro key from my laptop failed with windows 10 pro installation in a VM, as that key was apparently an 'upgrade' key. Sigh. It's such a mess. As if they suddenly realized that giving away the OS for free to many would cost them anything so they tried everything to limit the amount of installs.