Q. Is it true Windows 10 can now be installed using a Windows 7 or 8/8.1 key?

A. Yes but only from build 10565 onwards. Initially for a Windows 10 activation a Windows 10 key had to be used or an upgrade for Windows 7/8.1 had to be performed and only after that upgrade was performed a clean install on the same piece of hardware could be performed which would be automatically activated. With the new change it is now possible to perform a fresh install of Windows 10 and use a Windows 7 or 8/8.1 key.  This is documented at http://answers.microsoft.com/en-us/insider/forum/insider_wintp-insider_install/activation-faq-for-insider-preview-build-10565/68e759a2-abcc-4b16-8758-92a720a6efe9?auth=1 and will make it easier to perform future Windows 10 installations on new hardware.