I'm taking a look at the dnscheck.co service, which can email you when DNS queries' results do not match certain values.
This service may not do all that I need as it doesn't have range checks or IP & mask / prefix checks to cope with the rotation of the IPv4 addresses, all it has is a list of up to ten IP addresses that can be checked for equality and the results logical-ORed together, and ten isn't many. This is a bit silly given that the variation could be in a much wider range for all I know, even in a /24 or who knows what, something that they don't cope with. I've put in feature requests for range matches and masking.
I don't know if there is a better service anywhere that will monitor DNS changes.