You don't need the key. It's tied to your hardware.
In the installation (if you decide to reinstall), when it asks you to enter a key (IIRC, it'll ask twice), just choose "Skip" and carry on with the installation.
Once the installation is complete and your PC has a working Internet connection, Windows 10 will connect to the registration server, check your hardware ID and register automatically.
There's nothing else to do: no need to remember where the activation keys are, no need to extract the key from your current install, nothing.
Of course, if you do the in-place conversion, there's even less to worry about!