Dll Heaven: Multiple Versions One Assembly

Everyone wants their application to be backward compatible and if the application is based on a plug-in based architecture then such a feature can be a nice addition. When can such a feature be useful? Suppose we have a plug-in subsystem which we have upgraded to newer plug-in system and we want to support older systems. So if we want to load 2 different versions of a same assembly what do we need to do?