From the release notes

The server does not keep constant track whether a client is online (for network overhead reasons). The Symantec System Center viewer was changed to look at whether the client is overdue for checking in (every 60 minutes). This causes the client to show "Offline" within 60 minutes of going offline.

Hope you have tried the document

Moving Symantec Client Security 3.x or Symantec AntiVirus Corporate Edition 10.x clients from one parent server to another

http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2005041111454648?Open&src=tranus_ent_br&docid=2005041910541048&nsf=ent-security.nsf&view=docid

Your current SSC would give you a list of clients which are offline

make a note of them.

migrate as per above document, for the clients which are offline you need to run Makedrop.exe with new certificates.
Let me know if you have any questions

I had to reinstall the System Center for 10.1.8.8000, and now all the clients are showing up "off line". I have made the server the "Primary Server"....

I removed and install the SSC in the server with the same name and ip-address as before but only 8.x are now shown in the console.

All my 10.x clients does not shown on the console

Any ideas ??

Mike