The license applies
per copy. This is explicitly spelled out in The Wiindows 7 EULA. Section 2a:
One Copy per Computer.
You may install one copy of the software on one computer. That
computer is the “licensed computer.”
The license applies for One Copy on one computer, which would not include multiple copies on that computer, either with separate partitions or with Virtual Machines.
If i did decide to go dual boot which copy of windows 7 would be best to upgrade? My existing one or the 'new' one?
If I understand correctly, you want to Dual boot two copies of Windows 7, but only have one license. As we've shown, this would violate the EULA. With regards to upgrading one of those copies to Windows 10, that would violate the EULA as well;if you upgrade a Windows License, you cannot use the "original" license you upgraded without uninstalling the version you upgraded to. So in EULA terms a dual boot between Windows 7 and a Windows 10 install upgraded from Windows 7 requires two Windows 7 licenses.
You could image your current install, upgrade to Windows 10 and try it, and if you change your mind revert to the image you created to go back to Windows 7.