Rafiul27 :
Hi, I have the windows 10 free upgrade from windows 7.
If I ever need a reformat can I reinstall windows 7 after 29th July 2016, using my original windows 7 disc and then be offered a windows 10 upgrade like usual?
You will probably find that you don't get offered the upgrade after 29th July.
Your best bet would be to download a Windows 10 ISO image from Microsoft's website
here. The Media Creation Tool can create bootable DVDs, USB keys or simply save the ISO to your hard disk for you to burn off in future.
You will get a cleaner install by installing Win10 from scratch rather than upgrading from Win7. Your Digital Entitlement from your current Win10 installation should be recognised automatically if your hardware is unchanged or only slightly changed (new hard disk/SSD for example). There is a feature in the Anniversary Update due out on 30th July which should make re-activation more reliable by allowing you to associate your licence with a Microsoft account.