In your case there are 2 ways of restoring communication between the SEPM and the clients.
From the old client computer copy the Domain ID.
Then follow this process
To restore client communications without a database backup
Log on to the Symantec Endpoint Protection Manager Console.
Click Admin.
In the "System Administrator" pane, click Domains.
Under "Tasks", click Add Domain.
Click Advanced.
Open the disaster recovery text file, select and copy the domain ID and then paste the domain ID into the "Domain ID" box.
Click OK.
Repeat this procedure for each domain to recover.
Under "Tasks", click Administer Domain.
Click Yes on the "Administer Domain" dialog box.
Click OK.
Restart all of the client computers.
Note: The computers appear in the Temporary group.
If you use one domain only, delete the unused Default domain, and rename the newly created domain to Default.
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2007082112135948
The Second option you have is Call Support and ask for sylinkreplacer it will replace the sylink.xml on all your computers remotely.Once Sylink.xml is replaced it will connect back all your computers to the new SEPM.