vikram is right,those should help you out.

The SEP clients find the management server by looking at the Management Server List they are configured to use.  By default when you install the SEP Manager it creates a default Management Server List that contains the IP address of the manager and its name.  If you just change the address of the server SEPM will update this list but that will not change the server being referrence by those clients which have already been deployed.

You can create your own Management Server List and have the clients use it instead.  If you were to create a new Management Server List with only the name of the management server it would force the clients to resolve the IP address of the management server from DNS or NetBIOS and all should be well as long as you change DNS when you change the IP on the server. 

Another idea would be to create a new Management Server List with the IP address and name just like the default list but then add the new IP address that you will be changing the server to.  This way the clients will first try the old address, the new address, and then the name.

A Management Server List is not used until assigned to groups. To assign the list select Policies -> Policy Components -> Management Server Lists -> highlight the list to be assigned -> Assign the List... -> check the groups to use the list -> Assign.