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The in-place upgrade is the first choice for us to upgrade SCCM version, however you got the MP failure after the in-place upgrade, have we tried to troubleshoot and fix the issue follow some articles, such as:
https://www.anoopcnair.com/sccm-upgrade-resulting-in-mp-issue/
The first question:
For the backup and restore, the name of site server cannot be changed, you may check Jason's blog for details:
https://home.memftw.com/configmgr-site-backup-restore/
The second question:
That means you will build a new environment, and use migration, this should be the worst choice.
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