Now on Windows 10 .. and preparing for a future clean re-install

I have successfully "upgraded" ( via Window's Update) four machines to Windows 10. All good.

I now want to prepare a bit , for a clean install down the road. In the past, out of habit, I always created installation media ( USB/ISO etc) and kept a paper ( and electronic record of the Windows activation/product key ( the one it sometimes prompt you for after a clean install, especially after a hardware change. I have created Win 10 install media, but have a question about product activation keys.

I read the tutorial here Windows 10 - Clean Install - Windows 10 blog , and If I understood correctly,
-I wont need a product key for a future clean reinstall
-But, if I make a meaningful change to hardware, I may need to contact customer support to help with activation

So is there really no product activation key I can keep on file?

I do get some numbers ( different from WIn 8.1 number)s if I run various utilities right now, ie
-WinKeyFinder reports back that my 25 character key is T49TD-...
-Speccy and Belarc report back a different number, YTMG3-...
-and SLMGR at the dos prompt reports back a Partial Prodcut key of 8HVX7..

Are any of these valid, or am I at the whim of a future call to Microsoft customer support? Wont the line be busy with all those billions of users out there??

The product key is the same for everyone who upgrades and is the same as build 10240 which was the last release for the Insider Program, so no, there is none you need.

If you ever go back to Win 8.1 on any of these computers it will use the key embedded in BIOS so agsain, none you need to remember.

So if I am ever prompted for a key down the road ( eg clean install with updated hardware on my current machine) what 25 character key do I enter? Is it posted somewhere given that we all now use the same key? I am not familiar with "build 10240" but I did google it and got back W269N-WFGWX-YVC9B-4J6C9-T83GX. Is that the key to use?

You should not be prompted. If you are, click on Skip.

and, please, this question has been asked 100'sof times on this forum in the last 2 or 3 days; please use the search function before starting a new thread

So if I am ever prompted for a key down the road ( eg clean install with updated hardware on my current machine) what 25 character key do I enter? Is it posted somewhere given that we all now use the same key? I am not familiar with "build 10240" but I did google it and got back W269N-WFGWX-YVC9B-4J6C9-T83GX. Is that the key to use?

Just to be clear:

If you successfully upgrade to Win 10 on a PC, you can do a clean install on the same hardware using the free upgrade version. No key is given, or needed: click "skip" on the install screen.

I don't know how constant the hardware must remain before you'd have to activate by phone. For example: could you install on a new HD?

and, please, this question has been asked 100'sof times on this forum in the last 2 or 3 days; please use the search function before starting a new thread

True.

But don't jump on people anyway. It's more annoying than redundant new threads.