If the Free Upgrade period ends as Microsoft Marketing are aggressively stating (personally I don't think it will as Windows 7/Windows 8.1 Product Keys are too embedded with Windows 10 product Activation) then you have a mere 4 days left to upgrade if you haven't already.
This guide here gives detailed notes on A Clean Installation:
http://dellwindowsreinstallationguide.com/windows-10-threshold-2-free-upgrade-1-week-countdown/
I was assured in February by a Microsoft help desk chat person who gave me the iso download that I could use it after July 31st to upgrade just as long as I had my old serial number from Win7.