This may be the only guide you need if you are migrating from OpsMgr 2007 to OpsMgr 2012.  I was looking for the best how-to, step-by-step guide to work through this in my VM and happened on this today. Andrea Pichler does a great job, pulling in the proper resources and laying the whole process out on the page.
Check out the guide:  How-to upgrade to SCOM 2012 from SCOM 2007 – In-place Upgrade
I’ve already snipped this into an Evernote notebook.