Upgrading to ConfigMgr 2012 R2 is not a pain, but it could be. You really need to read and understand before you begin your journey.

The basic story is that you do an in-place upgrade and then fix “some” post items.

Just follow this fellow MVP and you should be good

Deployment Research (Johan Arwirdmark) http://www.deploymentresearch.com/Research/tabid/62/EntryId/117/A-Geeks-Guide-for-upgrading-to-ConfigMgr-2012-R2-and-MDT-2013.aspx

/mike