This week I received a question if OpsMgr 2007 R2 can be upgraded to 2012 SP1. The answer is Yes, that's possible! However you have the choice between an in-place upgrade or a side-by-side upgrade. Both options has their own benefits, disadvantages and configuration steps. Let's start to have a look at them.
Your first start is here: Microsoft TechNet
It's all about the existing configuration, how many OpsMgr servers there are, custom management packs, used Windows version, used SQL version and SQL Collation set, which must be SQL_Latin1_General_CP1_CI_AS. There are a few "Upgrade Process Flow Diagrams" which can be used during the upgrade process. They can be found HERE.
Another choice is about having pollution in the existing database. Do you want a new, nice and clean installation with new management packs and configuration, or do you want to use your existing environment with all custom management packs and configuration in it? Is history/trending important or do you want to start all over again? It depends on the situation I guess.
The upgrade to OpsMgr 2012 SP1 is a two way step. First a migration from OpsMgr 2007 R2 to 2012 is needed. Then the 2012 SP1 setup can be started for the next upgrade. To help you with the upgrade, a "Upgrade Helper Management Pack" is available on the OpsMgr 2012 installation media. Just use it, and follow the steps in the right order. More information can be found HERE.
For the first upgrade to 2012, the minimum cumulative update (CU) for OpsMgr 2007 R2 must be CU4. It's recommended to use the latest version available, which is CU7. Just download it HERE.
There are a few prerequisites which are needed before the upgrade: Just make sure there is a full backup of the Management Server and OpsMgr database, install DotNet Framework 4.0 and Report Viewer 2010 redistributable. Also make sure both Management Server and database is upgraded to the last patch level. A few manual steps are needed also, which can be found HERE. (Manual operations that must be performed after you update the root management server and the data warehouse).
After following all needed steps for prerequisites, the upgrade can be started. When ALL is configured right, this might not be a problem. In my LAB environment things went very well, but hey that's an almost clean environment. In a real situation I prefer a clean installation or side-by-side upgrade above an in-place upgrade. That way you can startover again and their is less risk for issues.
Hope it helps!