Hello.
1. start Knot DNS side by side with the PowerDNS but
on a different
port (say 53535)
2. create a firewall rule to redirect all the incoming traffic to the
port 53 to the 53535
3. shut down the PowerDNS instance, edit the Knot DNS configuration to
listen on both 53535 and 53 (reload doesn't interrupt the service)
4. remove the firewall rule
5. now you're running Knot DNS without any interruption in service
Or you can simply remove the PowerDNS instance from your anycast node,
reconfigure the instance to use Knot DNS, and reassign the instance back into
the anycast.
It's difficult to answer your question without providing more details on your
setup and definition of "maximum seamless efficiency or minimal temporary
disruption".
Jan