您好，
对于LTSC和SA您可能理解的不太贴切：
Windows server 2019属于LTSC长期服务频道版本，它是不会出现1809，1903等半年频道的版本号的，server2019对应的是server2016.。而Windows
Server, version 1903是属于SA半年频道版本，它是会随着feature
upgrade的安装而改变版本号，下次会使1909，20H1等。
如果想要从server2019升级到server1903，那只能clean
install
Windows Server 1903 is a semi-annual channel release that is a core only install.
https://docs.microsoft.com/en-us/windows-server/get-started-19/whats-new-in-windows-server-1903
Windows Server 2019 is a long term servicing branch release
https://docs.microsoft.com/en-us/windows-server/get-started-19/whats-new-19
Windows Server 1903 would need to be clean installed, there isn't a method to switch between long term and semi-annual
https://docs.microsoft.com/en-us/windows-server/get-started-19/servicing-channels-19
希望以上信息能帮助到您。
最好的祝福，
Kiki Shi
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