Hello
Thank you for your question and reaching out. I can understand you are having query\issues related to Windows 10 Pro upgrade to LTSC.
Microsoft refuses to allow any upgrade from Windows 10 to LTSC due to LTSC's application-related restrictions.
Hence I would suggest you to do a clean install of LTSC if you have valid license.
Reference:
https://learn.microsoft.com/en-us/windows/deployment/upgrade/windows-10-edition-upgrades
--If the reply is helpful, please Upvote and Accept as answer--