Windows 10 LTSC 2015 didn't block this in-place upgrade path.
In-place upgrade from Windows 7, Windows 8.1, or Windows 10 General Availability Channel to Windows 10 LTSC isn't supported. See the Windows lifecycle fact sheet for availability and service information. Work with your account representative if your current version of Windows is out of support. Windows 10 version upgrade: You can directly upgrade any General Availability Channel version of Windows 10 to a newer, supported General Availability Channel version of Windows 10, even if it involves skipping versions. However, applications and settings aren't maintained when the Windows edition is downgraded. Edition downgrade is also supported for some paths. These methods require that you input a license or product key for the new Windows edition prior to starting the upgrade process. Methods and supported paths are described on this page to change the edition of Windows.
If you're also migrating to a different edition of Windows, see Windows 10 edition upgrade. Migrating from one edition of Windows 10 to a different edition of the same release is also supported. This includes upgrading from one release of Windows 10 to later release of Windows 10. You can upgrade to Windows 10 from Windows 7 or a later operating system. This article provides a summary of available upgrade paths to Windows 10.
