Does Windows 10 use a Windows 8 key from UEFI?

The key should work, but you might have to enter it manually.

Please note:

Windows 8/8.1/Core -> Windows 10 Home
Wndows 8/8.1 Pro -> Windows 10 Pro

Thank you.

Okay, thank you all. I seriously hope it just works – because there is nothing to enter manually (no key sticker like with Win 7 and older)

Might take a few days until I get the new SSD but I will post an update then.

you can get a piece of software that can extract the key and tell you what it is, i forget what it's called but it's an easy search away

RWEverything maybe.
You can install Windows 10 without entering a Key and do the search after the Installation.

i just did a quick google search and there's this tool called "OEMKey" or "Windows 10 OEM Product Key Tool", it's a quick download and run tool that spits out your key in plaintext, doesn't work if there is no key in the bios and shows a "Unable to open MSDM table! This PC does not contain an embedded product key" error

woah, so my windows 7 product key will work with windows 8.1 and 10?

@Missingphy If you never used it for a Windows 10 installation,yes.

Sorry for the delay. The SSD arrived late.

The easiest answer was correct and sufficient: It just works. Windows 10 Home used the embedded Windows 8 (Core) key automatically and activation was successful seconds after connecting the Ethernet cable.

Thank you all for your answers! You gave me much more information than I had asked for. Although everything worked, I still will extract the key with OEMKey. Somehow this gives a better feeling.

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The hardest part was creating a bootable USB medium because the version "1903" ISO does not fit on single layer DVD and install.wim is just above the 4GB limit of FAT32.

I have had the same issue with Windows 10 and trying to find a flash drive or DVD big enough to hold it. Glad you got it working!