What happens when I setup the new SubCA with the existing private from Sub1?
this is not the right way to do things. If you want to retain the key, then you have to migrate existing CA with all the configuration and CA database to another box. Refer to this document on migration scenarios and migration steps: https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-r2-and-2012/dn486797(v=ws.11)