Hi,
Thank you for coming Microsoft Q&A forum.
Agree with above reply. Based on my experience, it's no need to add enablement package to existing task sequence to upgrade the OS.
1.If you would like to upgrade from Windows 10 1903 to 1909 via task sequence, it's recommended to add an Operating System Upgrade Package and use in-place upgrade task sequence. For more details, please refer to:
Windows 10 1909 Upgrade Using SCCM Task Sequence|ConfigMgr
2.About feature update to windows 10 1909 using Enablement Package(for Windows 10 1903), please refer to:
[ConfigMgr] Deploying Windows 10 1909 Update using Windows 10 Servicing on SCCM]2
Please note: The links are not from Microsoft, just for your reference. Thanks for your time.
Best regards,
Simon
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