Move Accounts to Another Server with No Downtime?

Hello, I have a situation that it's a bit confuse for me. I have 2 WHM cPanel Servers and one VPS with Only DNS. I have a domain name exampledomain.com for main domain to all these 3 servers. Example: Main WHM Server with DNS of the exampledomain.com -> hostname1.exampledomain.com Second WHM Server -> hostname2.exampledomain.com DNSONLY setup -> ns21.exampledomain.com I want my Main WHM Server that have the main domain exampledomain.com and DNS records to format it and install new centos and cPanel fresh installation. The problem is the time that I make this work my second server that have hostname2.exampledomain.com will not resolve the mails and sites of websites that are there?? How is possible to make my work with no downtime to my second WHM Server? See below the structure: Main WHM Server {That I want to shutdown and re-install} I have it with cluster to DNSonly hostname.exampledomain.com ns21.exampledomain.com {Write-only}
Main WHM Server DNS records of the domain that sharing: DNS - Records of my Main WHM Server exampledomain.com 300 IN NS ns1.exampledomain.com exampledomain.com 300 IN NS ns2.exampledomain.com ns1 14400 IN A 123.123.123.123 {Main WHM Server IP} ns2 14400 IN A 123.123.123.123 {Main WHM Server IP} ns100 14400 IN A 123.123.123.111 {Other WHM cPanel Server} ns200 14400 IN A 123.123.123.222 {My OnlyDNS as second nameserver on my other cPanel Server} ns21 14400 IN A 123.123.123.222 {My OnlyDNS hostname and nameserver on MyOnlyDNS vps setup} exampledomain.com 14400 IN MX 0 exampledomain.com exampledomain.com 14400 IN A 123.123.123.123 hostname2 14400 IN A 123.123.123.111 {Other WHM cPanel Server Hostname}
From intodns: exampledomain.com Results: ns1.exampledomain.com 123.123.123.123 ns2.exampledomain.com 123.123.123.123
I want to mention that all the records also is on ns21.exampledomain.com Also I have setup the second WHM server as: hostname2.exampledomain.com ns21.exampledomain.com {Write-only}
Can advice me how I can make no downtime of my second server that have the same domain in hostname and ns records? Thank you in advanced!