No, the services transfers should be seemless. Before switching the domain's name servers to our name server, you must setup the DNS for the domain in our system to match it's currently DNS setting. Things that you should setup at DNS may include MX, A Records(Hostname) for the domain name. Assume the current name server of the domain is ns1.current.com and your domain name is mydomain.com, then you can find out the current setting with the following DOS/Linux commands.
To find MX (mail exchangers) of your domain name:
C:>> nslookup -type=mx mydomain.com ns1.current.com
To find the IP for your domain:
C:>> nslookup mydomain.com ns1.current.com
To Find the IP of your web site:
C:>> nslookup mydomain.com ns1.currently
C:>> nslookup www.mydomain.com ns1.currently
You then setup the DNS for your domain to match your current DNS settings. Then you can switch the name servers of the domain to our name servers ns1.dnsexit.com, ns2.dnsexit.com, ns3.dnsexit.com, ns4.dnsexit.com. It will not introduce any downtime to your domain. At the first few hours switch, the DNS queries to your domain will slowly migrated to our name servers to resolve. Eventually, queries to the old name servers will stop.