We have two dns servers for our zone (nw 6.5 sp5) which are authoritative
ones for the zone and one of them is designated primary server. As this
designated primary server is running as the ftp server at the same time, we
want to pronounce the second dns server as the designated one.

Is it enough to change those roles through DNSDHCP Management Console (or
you mean iManager is better to do it) in order to move primary dns server
from one server to another (which is secondary one now) or we have to do
something more?