There's several things to consider when changing to a new IDE and then moving your applications to a new framework. Depending on if your developing for yourself in terms of development or for a company which will require stages to complete.
Converting to VS2012 only - From a previous version you will only have to open the old vs2008 solution in VS2012 and it will convert the project files to the new IDE version of VS2012 automatically. There shouldn't be any isssues after that unless your working with versioning systems, then you need to make sure you save the new VS2012 project and check it in. From there you will notify the team they will have to get an update before opening in VS2012.
Changing your Framework - You may have some complications due to some of your current libraries may have some obsolete references in it and you will need to make sure you do some rather intense testing to make sure your application is not impacted when using those old references in the new framework.
Can it be done, yes, but you need to keep a copy of both the VS2008 version and the previous framework before doing so just to avoid any down time or any complications that will cause others frustration.