Interface Library Versioning - Breaking Changes

I currently have a C# project which uses plugins and has a fairly common approach to plugin handling: an IPlugin interface is stored in a dll which is linked in a tranditional dynamic way. The host app looks for class libraries exporting classes exposing this interface and loads them via reflection at run time.

The dll containing the interface also contains helper classes, for updating plugins, providing abstract base classes and so on.

My question is, what does it take to break the interface between my host and plugin assemblies? In other words, if I compile and distribute the开发者_StackOverflow host app and then distribute plugins that have been linked with a later version of the plugin dll (in which helper classes have changed, but IPlugin is defined in exactly the same way), will the host still pick up the plugins? How much of a change do I need to make to the plugin library before IPlugin is considered a different "type" by the reflection methods I am using?

If the assembly isn't loaded by a specific version than I would say the only breaking changes you will really encounter are when you change the interface contract. If you are just changing helper classes it shouldn't be a problem.