Upgrading and downgrading an application is always a delicate process.  The easiest part is getting the application to use the new assemblies.  It is the testing you have to do afterwards to make sure there are no problems once making an upgrade or downgrade that can take some time. 

If you place this question in the scope of Infragistics controls and Windows Forms, there are a few things to consider.  We do maintain backwards compatibility so going back and forth should not be too much of an issue.  You just have to change the references to point to the new version and recompile.  When downgrading, there are some cases where classes did not exist in 6.1 that exists in 7.2 (ribbon for example) and you will have to remove those elements to get your application to compile in the older version.  When going up in a version, you run the risk of functionality being changed, an introduction of a bug or behavior change to fix another bug.  With those possible complication aside, there are numerous ways to start step 1 listed in our help article: