Server 2019 v1809 is a long term servicing branch channel. To switch to semi-annual channel you must clean install it. Also note the SAC versions are typically core only.
https://learn.microsoft.com/en-us/system-center/ltsc-and-sac-overview
The next LTSC version of windows server is Server 2022
https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server-2022-preview
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