If you do the upgrade, you will not have a copy of the install files. You will need to download them in the future from Microsoft if you want to do a clean install of Win10.
I think Microsoft recently changed it such that you can immediately do a clean Win10 install with an eligible Win7/Win8 key. I’m not 100% on that though. Doing the upgrade on an existing OS to link your hardware to the Win10 key is definitely the safe way.
Once you do the upgrade, you can install Windows 10 freely on that system. Over and over every few months if you want.
Or do an immediate bootable backup to a second internal hard drive after installing the Win 10 upgrade. Then remove it from the box and put it on your closet shelf.
There’s an inbuilt option to ‘refresh’ or ‘reset’ your PC - details here - the ‘reset’ option is essentially a wipe and reinstall of the OS; the ‘refresh’ option is like running the upgrade all over again.