Please check whether the client communicates to your SEPM 12 server instead of SEPM11?

sylink replacer tool for sep 11 and sep 12.x are different

Process of replacing sylink.xml are different in version 11.0 and 12.1 and hence the tools are also different.

In case if your SEPM is 12.1 and the clients are 11.0 you would need a 11.0 sylink replacer.

...from the description you provided in the OP, you've successfully redirected your SEP11 clients to your new SEP12.1 server using SyLinkReplacer, and that approximately one day after a successful redirection, the client appears as offline on the new 12.1 SEPM.

Have I got that right?

If that is the case, you may find the below articles useful.  These focus on troubleshooting communications bewteen the client and server.

http://www.symantec.com/docs/TECH160964

http://www.symantec.com/docs/TECH105894

http://www.symantec.com/docs/TECH95789

If the communications testing comes back fine, then it may also be worth reviewing the SyLink.xml file again.

Hello,

I believe you are transferring all the SEPM 11 clients to SEPM 12 clients and all the newly sylink replaced clients are turning ack to offline state, correct?

Questions - 

1) Does the new SEPM 12 server carry a dedicated IP address?

2) Is there a Firewall / Proxy between clients and servers?

Would it be possible for you to upload a sylink.log from the 1 of the SEPM 12 machine which is offline.

Check this Article on how to collect the logs:

http://www.symantec.com/docs/TECH104758

Hope that helps!!