Agreed. Two big challenges. But to be clear—we're talking here only about zone file management, not registration transfer (if that's what you mean by migration).

In the case of a large-scale, multi-domain authoritative nameservers assignment change, only the DNS provider would be able to facilitate the process programmatically. And at that kind of scale (i.e. hundreds of domains), one would hope verification would not only be obtained through, but require some form of human-to-human contact coupled with thoroughly documented scope.