IIRC, someone posted a Python script that lets you change the shading rate of everything in a scene.

But you may not want to do that.  It will make rendering really, really slow.  I usually set the shading rate individually.  For example, hair often requires a lower shading rate than anything else, because of the kind of texture it has.   A really low shading rate may be needed for a closeup of Vicky's face, but not for a full-body shot.  Etc.