Hello.
You can migrate admin server by the following command. This command must be executed on the current admin server.
> tell certmgr quit
> load certmgr -MIGRATETOSERVER <New Admin Server>
But there is the following defect. You can use this command in Domino 12.0.2 FP3 or later.
https://support.hcl-software.com/csm?id=kb_article&sysparm_article=KB0105828
Regards,
Shigemitsu Tanaka