change name server

[Rossomak]

Hi, I like to ask you, if my ispCP running long time on name server - mydomainA.com and I need to change name server to mydomainB.com, how can I change this without reinstalation.
On ispCP running allready cca 20 webs and cca 50 emails, and I dont want to reinstall all webs on hosting.
I use many years nameserver mydomainA.com, now I needed for something else, so I registered mydomainB.com, and I dont know how to change it.
Thank you
Rossomak