So process should be following for the Windows before "Fall update":
- download ISO or create USB with Media Creation Tool
- run setup from inside Windows which will start in-place upgrade
- once you upgrade "in-place" and connect to net, your Windows 10 will be activated using underlying Win 8.1 key, you don't hve to do anything, but you can check in "System" at the bottom if it's activated
- once everything is activated you can format your HDD, do clean install, or do a refresh/reset of the Win10 installation
.Now, AFTER the "Fall update" it was said that if you create NEW bootable image (ISO/USB) using Media Creation tool, and boot from DVD/USB, you'll be asked a key and you can than directly enter the key of Windows 7/8/8.1/10 whichever you have, as long as it's same edition (Home -> Home, Pro/Professional -> Pro etc). Thing is, I've just read that Fall update was pulled and unsure if it was returned, and I'm not sure which version of Win 10 Media Creation tool currently creates :-( And I haven't tested this procedure yet. So if you're in no hurry you can wait a bit to make sure Microsoft updates all tools from build 1024 (RTM) to build 10586 or newer (Fall update 2015/11, version 1511, or newer).
I'd post a few links but you'll have to Google yourself, I'm still "limited account" here so can't post links
EDIT: oh, and if you don't have license sticker on your laptop/PC, you can extract key with one of many key retrievers, again, you'll need to google for it, sorry :/ but Magic Key was one and Nur soft has one as well.