By migration I think you mean installing IM Manager on a new server and using the existing IM Manager database.  Then shutting down your old server.  This is a straightforward process.

* Set up new server.
* Install IM Manager on new server.  When the installer runs the installer will ask you if you want to use an existing database or a new one.  You will choose existing database. 
* Then test your new server.   If this is for public IM you can test against the new server using host files: http://service1.symantec.com/support/ent-gate.nsf/854fa02b4f5013678825731a007d06af/949a398596db626888257341006a9e18?OpenDocument.
* Once you are happy you can start using your new server.  If this is a public IM server you will need to point public IM clients to it.  Here are the DNS entries you need to change: http://service1.symantec.com/support/ent-gate.nsf/854fa02b4f5013678825731a007d06af/0201fa48d7c6677f88257341006aa0ad?OpenDocument.
* There still may be information on the system dashboard for the old server.  Use the following KB to clear them out: http://service1.symantec.com/support/ent-gate.nsf/854fa02b4f5013678825731a007d06af/a364063fc4afb13c88257341006aa0a8?OpenDocument

Ben