An OEM version can only be installed once and if you change the processor and mainboard, that's it, or am I wrong?
The versions you get at MMOGA and co. win10 for 15 to 20 Euros are exactly such OEMs and you can't use them anymore when you change hardware.

Question

An OEM version can only be installed once and if you change the processor and mainboard, that's it, or am I wrong? 
The versions you get at MMOGA and co. win10 for 15 to 20 Euros are exactly such OEMs and you can't use them anymore when you change hardware.

Answer

No, an OEM version can be installed more than once and an OEM version does not expire after a hardware change. Depending on the change, Windows must be reactivated with the licence key.

https://www.heise.de/ratgeber/FAQ-Windows-kaufen-4920043.html

Answer

https://support.microsoft.com/en-us/windows/reactivating-windows-after-a-hardware-change-2c0e962a-f04c-145b-6ead-fb3fc72b6665#ID0EBD=Windows_11 explains how to take a licence with you when you change hardware. Requires a Microsoft account.

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Question about Microsoft Windows 11 Pro

An OEM version can only be installed once and if you change the processor and mainboard, that's it, or am I wrong? The versions you get at MMOGA and co. win10 for 15 to 20 Euros are exactly such OEMs and you can't use them anymore when you change hardware.