nChild should not remove the old key while there are still servers
handing out the old DS RR.
nThe new DS will need to be distributed to the slave servers
umax time set by the SOA expiration time
nThe old DS will need to have expired from
caching servers.
uSet by the TTL of the original DS RR.
nYou (or your tool) can check for the master and slave to have
picked up the change.