This is pretty easy...
By default, all slide notes are assigned the Presenter Notes style.
Just like in your slide content (and in other apps such as word processors), styles let you assign constant 'look and feel' to your documents - all slide titles, for example are assigned the 'Title' style, and changing the definition of the style automatically applies that change to everywhere that style is used.
Same thing for presenter notes - they're all assigned the Presenter Notes style, so redefining that style should apply to all notes on all slides.
The trick is to look at the Inspector -> Format when editing presenter notes:

When you edit the currently-selected presenter notes, the Style Name box will display an 'Update' button:

Clicking this button will update the style definition and apply the changes everywhere that style is used in the document.