Upgrade Win 10 Pro to Win 10 LTSB

In Place Upgrade To LTSB 2016
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Change EditionID and ProductName at registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion
EditionID = EnterpriseS
ProductName = Windows 10 Enterprise S

Then just run setup.exe from LTSB ISO (extracted to a temp folder somewhere).