Dear Mukul!
1) Do you have many changes in your zones, or is it unproblematic if you are not able to change anything for a few days?
I ask because my idea is to add a new name for the new primary and secondary DNS servers build with NSD while leaving your old setup as it is. That means, if your current names are "ns1.example.com" and "ns2.example.com", you would add "alpha.example.com" and "bravo.example.com", build with DNS. Once you think all runs fine, you would change the DNS server names for your domain. In case anything fails and you are not able to fix that in a timely manner, you can switch back. The names "ns1" and "ns2" will be always the old DNS servers and at some point you shut them down.
I think you already know that of course, but with tools such as dig from dnsutils (Debian) you can always easily check if your new NSD nameservers responds correctly:
# dig -t A example.com @PRIMARY_DNS_IP_ADDRESS_OR_HOST
# dig -t A example.com @SECONDARY_DNS_IP_ADDRESS_OR_HOST
2) How do you usually edit your zones?
--Kaulkwappe