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I never do an upgrade on my existing production hardware. I have 2 or 3 sets of virtual machines that I use for the upgrade testing and then production upgrade.
Build your 13.0.2.1 system on the RHEL 8.8 and then import the database files, copy the file vaults over and finally run UpgradeManager. That should give you a clean running 13.0.2.1 system.
There are some changes in the usage of the LDAP that came into play at Windchill 12.0.1, so not sure how that will be handled in the upgrademanager. always do a test upgrade before doing the production one. I did a production Windchill 13.0.2.1 upgrade last week from 12.0.2 but on Windows Server 2019.