Interesting question, and based on the strict definition of the open closed principle I can see where you're coming from.
I have come to define the open-closed principle slightly differently, and this principle I think should apply, and that is to apply it far more broadly.
I like to say, all my classes (as a whole) involved in the application should be closed for modification and open for extension. So the principle is that if I need to change behaviour and/or operation of the application, I do not in fact modify a class but add a new one and then change the relationships to point to this new one (depending of course on the size of the change). If I'm following the single responsibility and utilising inversion of control, this should occur. What then occurs is that all changes come to be extensions. They system now can both act in the former way and in the new way and changing between them = changing a relationship.
