Design using MVP pattern

First, why should the presentation layer work in two different presentation models?

You mean the controller?

I can see this:

Model <shared>
Controller <shared>
View - one instance for web, one instance for web form

That makes more sense to me.

I'm also assuming your changes are to "something" not just random fields, but fields that belong to an object.

So why not maintain the original object state in the controller:

Controller<T> where T: class

T _state;

Have a typed args, something like:

public class TypedArgs<T> : EventArgs where T: class
{
public T Value { get; set; }
}

Create an interface for your views:

public interface IView<T> where T: class
{
event EventHandler<TypedArgs<T>> Changed;
}

then you can raise Changed with the new entity in the view, and have the controller register. Just pass in IView to the controller so it doesn't care if it's a user control or a web form, and when Changed is raised, it can compare e.Value to the stored state and decide what to do.