Hierarchy Simplification and Secondary's

Everyone should look closely at their hierarchy when performing their migrations from 2007 to 2012. 

If you are utilizing Secondary as content distribution and you wish to continue to use the Secondary as such and at the same time maintain the data on the DPs you mush have a new 2012 Secondary setup to take over the role.

DPs attached to the Secondary must be upgraded first.  During the upgrade process the Secondary role and MP role is removed and the server becomes a DP.  For this you must have a 2012 Secondary ready.  During the upgrade process you attach the DPs to the New Secondary.

Planning for Content Deployment During Migration to System Center 2012 Configuration Manager
http://technet.microsoft.com/en-us/library/gg712275.aspx

Now you can continue to use the secondary as content distribution system without the need to redistribute the content.