In SCCM 2007, administrators use secondary sites in a few scenarios. One of the most common uses is to control the flow of data across slow or unreliable WAN links. With SCCM 2012’s new throttling and scheduling capabilities, content control can be accomplished with distribution points alone. During your migration, sharing a secondary site distribution point is done just as you would share any other distribution point, but upgrading is a little different. When you upgrade an SCCM 2007 secondary site to an SCCM 2012 distribution point, SCCM first uninstalls the secondary site. Then, at the next data-gathering interval (the default interval is four hours), it upgrades the distribution point to an SCCM 2012 standard distribution point.
There were some problems in the early days of migration. When upgrading an SCCM 2007 secondary site to an SCCM Training 2012 (no service pack) distribution point, the presence of the PXE Service Role could prevent the upgrade. This problem was resolved in SCCM 2012 SP1 and should no longer be an issue. If you haven’t installed SP1 and you encounter this scenario, you need to remove the PXE Service Role prior to upgrading the distribution point.