Many are waiting for ConfigMgr 2012 SP1 before they actually go from ConfigMgr 2007 to ConfigMgr 2012. If you for some reason have a Group Policy that assigns clients to a specified Site code you will hit a barrier migrating your clients with the new Agent. The installation itself will go fine but it stops there.
The Group Policy tattoo’s the Assigned Site Code locally on the machines that are hit by the policy.
What you can do from here is either change the Site Code in the same, or a new Group Policy hitting the migrated machines, to reflect the correct Site Code.
Delete the regkey’s holding the forced Site Assignment and let the Agent discover its Site Assignment automatically as you normally would with Boundaries(AD Sites, IP Ranges etc.).
REG delete "hklm\SOFTWARE\Microsoft\SMS\Mobile Client" /v GPRequestedSiteAssignmentCode /f
REG delete "hklm\SOFTWARE\Microsoft\SMS\Mobile Client" /v GPSiteAssignmentRetryDuration(Hour) /f
REG delete "hklm\SOFTWARE\Microsoft\SMS\Mobile Client" /v GPSiteAssignmentRetryInterval(Min) /f
Once either of the two solutions have been put into effect you can go into your ClientLocation.log and LocationServices.log files and check up on the new Agents connecting to the proper Site Code.
Note: the LocationServices.log file will tell you if the Site Code the Agent are trying to connect to is Group Policy enforced.
(Example: “Found Assigned Site Code <PS1> by Group Policy”)