I think the method of doing a 'full' install has changed as before you had to upgrade first before doing a full install. It would not let you do a full install of Win10 until you done the upgrade, not sure why but it may have had something to do with the product keys as now when you get the upgrade you dont get a product key. Instead Win10 is activated with a 'digital entitlement'.
A little off topic, but when I installed windows 10 on my new build recently I didn't enter my win7 key as I was still using it in my old computer. I continued using it like this for few days untill I moved some of old components to the new computer and then transferred the win7 key. Everything activated perfectly fine, but was wondering to myself, how long could I have used it in an unactivated state? Ther wasn't any time limit or anything, nor any reduced functionality I could notice.