You can change the upgradematrix.xml from the Enterprise 2021 LTSC ISO and try to upgrade with it and next switch to IoT Enterprise 2021 LTSC. https://forums.mydigitallife.net/th...prise-n-ltsc-2019.76325/page-289#post-1638636 + https://forums.mydigitallife.net/th...-8-1-pro-to-ltsc-iot-2021.85266/#post-1732401 Best would be to perform a clean install of the desired SKU and set all up fresh.
Pretty simple use the LTSC 2021 ISO Change (your current registry) \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\editionID to EnterpriseS and \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProductName to Windows 10 Enterprise LTSC 2021 (or 2019) Once finished just insert the IoT serial using slmgr /ipk No need to fresh install, it's just stupid.
You upgrade to LTSC, then (if you really want IoT) use the command slmgr /ipk <IoT serialnumber> No idea on how put it simpler than this.
I would add a "+1" to the idea of doing a clean install. Upgrading from Windows 8 to 10 is a pretty big step, and I wouldn't want any old junk left around. The only exception would be if you have some particular 3rd party software installed and you no longer have the ability to re-install that software. As in, don't know the license key or don't have and can't find an installer (company out of business, etc).
Isn't a big step at all. In the worst case all you have to do is to create a new user and move your data there. For sure way less time consuming than a stupid format and reinstall. Obviously there are exceptions, but those are there just to confirm the rule