Hello there,
LTSC2019 is exactly the latest version of Windows 10 LTSC. For now there is no higher version of LTSC to upgrade.
If you would like to upgrade from Windows 10 LTSC to Windows 10 Semi-Annual Channel(SAC), provided that you upgrade to the same or a newer build version and use the iso installation.
“You will need to use the Product Key switch if you want to keep your apps. If you don't use the switch the option 'Keep personal files and apps' will be grayed out.”
https://learn.microsoft.com/en-us/answers/questions/54366/update-windows-10-enterprise-ltsc-%D0%B4%D0%BE-windows-10-en.html
You can check the upgrade path in beneath link:
https://learn.microsoft.com/en-us/windows/deployment/upgrade/windows-10-upgrade-paths
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You may find further information in similar cases:
“Update Windows 10 Enterprise LTSC до Windows 10 Enterprise”
https://learn.microsoft.com/en-us/answers/questions/54366/update-windows-10-enterprise-ltsc-%D0%B4%D0%BE-windows-10-en.html
“Update Windows 10 Enterprise to latest Version”
https://social.technet.microsoft.com/Forums/en-US/82465fda-ed89-41ca-960f-1080cd62dbea/update-windows-10-enterprise-to-latest-version
“I have Windows 10 Enterprise, but the new Updater tool says I don't”
https://answers.microsoft.com/en-us/windows/forum/windows_10-windows_install-winpc/i-have-windows-10-enterprise-but-the-new-updater/905b67c1-1c5c-4000-af7f-4fedfd2c1fe7
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Best Regards,
Samson Peng