The toolkit's documentation states migration from cPanel to ISPConfig requires the same underlying OS. I'll be moving 30 accounts from cPanel to ISPConfig, has anyone tried going cPanel/Centos 7 to ISPConfig on C7, then migrating ISPConfig to Debian 10? My clients are running email, static websites & WordPress.
That's not the case. You probably mixed up ISPCopy with the Migration Tool (both programs are parts of the Migration Toolkit), ISPCopy requires the same OS while the MIgration Toolkit does not has such a requirement.