What you need to do is quite easy, really:
Firstly, sync all of the dns data across
Secondly, change the ips to your one or two (in the example case ns19/20) nameservers to the new "clustered" nameservers. Change it in DNS AND at glue (your registrar).
Your customers shouldn't have to do anything at all if this is done properly, and the cluster is maintained properly.
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