Did the free upgrade this morning from Windows 8.1 and it works perfectly. 100% free.
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'.
However, before upgrading this morning I researched the process on the net and everywhere said you do not need a product key when doing a full install, you can just click 'Next' on the product key page and leave it blank.
I found this not to be the case when I did it, I still had to put in my Window 8.1 product key for it to proceed. The 'Next' button was greyed out unless I filled the product key box with a valid serial number. Microsoft implemented this method so that you can use your old Windows 7/8 product key but when people say you do not need a product key to do a full install is not true. You need to have your Windows 7/8 product key to hand or you are stuck on the install screen.
I think the download for Win10 was about 3gb, copy the ISO to a DVD or copy the setup file to a USB key. Either way, the setup app will guide you through the process.