1) You can't TRANSFER a license (even if possible) once you upgrade to W10. That will invalidate the license completely.
2) *I recommend WAITING if you want, then install Windows 10 from scratch (after July 31st).
As it stands now, if you install W7 then you can register and W10 will download and automatically install itself which may or may not go perfectly. It also may not delete backup files but we'll see.
I assume there will be a way to download the W10 image, burn to DVD, and install manually then input your W7 activation code but unfortunately Microsoft isn't clear on this. (I want to do a CLEAN install as well for my existing setup.)